Semiconductor device and method of fabricating the same

ABSTRACT

Obtained are a semiconductor device which can prevent diffusion of an impurity contained in a gate electrode and a method of fabricating the same. In this semiconductor device, a gate oxide film and a P+-type gate electrode which are formed on a P-type silicon substrate are doped with nitrogen.

This application is a divisional of application Ser. No. 08/880,508filed Jun. 23, 1997 U.S. Pat. No. 6,300,664 which is a continuation ofSer. No. 08/298,099, filed Aug. 30, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method offabricating the same, and more specifically, it relates to asemiconductor device which can improve element characteristics by anitrogen implantation technique, and a method of fabricating the same.

2. Description of the Background Art

In general, it is known that source/drain regions of a MOS transistorare formed with shallow junction planes in order to suppress a shortchannel effect of the MOS transistor. In order to suppress a shortchannel effect of a P-channel MOS transistor (hereinafter referred to as“PMOS transistor”), it is effective to employ an electrode which isdoped in a P type as an electrode material for the PMOS transistor. Inorder to suppress a short channel effect of an N-channel MOS transistor(hereinafter referred to as “NMOS transistor”), on the other hand, it iseffective to employ an electrode which is doped in an N type as anelectrode material for the NMOS transistor. A dual gate CMOS transistoris proposed in relation to application of these effects to a CMOS(complementary MOS) transistor which is formed by NMOS and PMOStransistors. In such a dual gate CMOS transistor, a gate electrode whichis doped in an N type is employed for the NMOS transistor, while a gateelectrode which is doped in a P type is employed for the PMOStransistor.

Conventional methods of forming source/drain regions of PMOS transistorswith shallow junction planes are now described. FIG. 145 is a sectionalview for illustrating a first conventional method of formingsource/drain regions of a PMOS transistor with shallow junction planes,and FIGS. 146A and 146B are sectional views for illustrating a secondmethod. Referring to FIG. 145, element isolation oxide film 7, a gateoxide film 2, a gate electrode 3, an oxide film 4 and side wall oxidefilms 5 are formed on a main surface of an N-type silicon substrate 1through an ordinary process in the first method. Thereafter the elementisolation oxide film 7, the oxide film 4 and the side wall oxide films 5are employed as masks to ion-implant boron fluoride ions (BF₂ ⁺) havinga larger mass number than boron ions (B⁺) into the N-type siliconsubstrate 1. Thus, source/drain regions 6 are formed with shallowjunction planes.

In the second method, element isolation oxide film 7, an oxide film 4and side wall oxide films 5 are employed as masks to implant siliconions (Si⁺) or germanium ions (Ge⁺) into an N-type silicon substrate 1,as shown in FIG. 146A. Thus, ion-implanted regions of the N-type siliconsubstrate 1 are brought into amorphous states. Thereafter boron ions(B⁺) are implanted into the N-type silicon substrate 1, as shown in FIG.146B. Thus, source/drain regions 6 are formed with shallow junctionplanes. In the second method, the silicon ions or germanium ions areimplanted in order to prevent a channeling phenomenon of the boron ions.

When the source/drain regions 6 are formed with shallow junction planes,however, the source/drain regions 6 are disadvantageously increased insheet resistance. To this end, generally proposed is a countermeasure ofproviding titanium silicide films 8 having low resistance on surfaces ofthe source/drain regions 6, as shown in FIG. 147.

FIG. 148 is a sectional view showing an exemplary conventional dual gateCMOS transistor. Referring to FIG. 148, an N well 13 and a P well 14 areadjacently formed on a main surface of a P-type silicon substrate 11.Further, element isolation oxide film 12 are formed on the main surfaceof the P-type silicon substrate 11 at prescribed spaces. P-typesource/drain regions 21 are formed on a main surface of the N well 13 ata prescribed space, to hold a channel region 10 therebetween. A gateelectrode of a polycide gate structure formed by a polysilicon film 16which is doped in a P type and a tungsten silicide film 18 which isformed on the polysilicon film 16 is provided on the channel region 10,through a gate oxide film 15. An oxide film 19 is formed on the tungstensilicide film 18. Side wall oxide films 20 are formed on side surfacesof the polysilicon film 16 and the tungsten silicide film 18.

On the other hand, N-type source/drain regions 22 are formed on a mainsurface of the P well 14 at a prescribed space, to hold a channel region10 therebetween. A gate electrode of a polycide gate structure formed bya polysilicon film 17 which is doped in an N type and a tungstensilicide film 18 is provided on the channel region 10 in the P well 14,through a gate oxide film 15. An oxide film 19 is formed on the tungstensilicide film 18, while side wall oxide films 20 are formed on sidesurfaces of the polysilicon film 17 and the tungsten silicide film 18.

FIGS. 149 to 157 are sectional views for illustrating a fabricationprocess for the conventional dual gate CMOS transistor shown in FIG.148. With reference to FIGS. 149 to 157, the fabrication process for thedual gate CMOS transistor shown in FIG. 148 is now described.

First, element isolation oxide film 12 are formed on a main surface of aP-type silicon substrate 11, as shown in FIG. 149. Further, an N well 13serving as a PMOS transistor forming region and a P well 14 serving asan NMOS transistor forming region are formed on the main surface of theP-type silicon substrate 11, to be adjacent to each other.

Then, an oxide film 15 a is formed to cover the N well 13 and the P well14, as shown in FIG. 150. A polysilicon film 9 is formed on the oxidefilm 15 a and the element isolation oxide film 12 by CVD, and a tungstensilicide film 18 a is formed on the polysilicon film 9 by sputtering.

Then, the PMOS transistor forming region is covered with a resist film25, as shown in FIG. 151. The resist film 25 is employed as a mask toimplant arsenic ions (As⁺) into a portion of the polysilicon film 9located on the NMOS transistor forming region. Thereafter the resistfilm 25 is removed.

Then, the NMOS transistor forming region is covered with a resist film26 as shown in FIG. 152, and this resist film 26 is employed as a maskto implant boron fluoride ions (BF₂ ⁺) into a portion of the polysiliconfilm 9 located on the PMOS transistor forming region. Thereafter theresist film 26 is removed. An oxide film is formed by CVD, andthereafter this oxide film, the tungsten silicide film 18 a and thepolysilicon film 9 are patterned in the form of gate electrodes byphotolithography and anisotropic etching. Thus, the oxide films 19, thetungsten silicide films 18 and polysilicon films 16 a and 17 a areformed as shown in FIG. 153. Thereafter an oxide film is formed on theoverall surface by CVD, and this oxide film is etched back. Thus, sidewall oxide films 20 are formed on side surfaces of the gate electrodes,as shown in FIG. 154.

Then, the PMOS transistor forming region is covered with a resist film27, as shown in FIG. 155. This resist film 27 is employed as a mask toimplant arsenic ions into the NMOS transistor forming region. Thereafterthe resist film 27 is removed.

Then, the NMOS transistor forming region is covered with a resist film28 as shown in FIG. 156, and this resist film 28 is employed as a maskto implant boron fluoride ions into the PMOS transistor forming region.Thereafter the resist film 28 is removed. Heat treatment is carried outfor activating the ions as implanted. Thus, a polysilicon film 16 whichis doped in an N type, a polysilicon film 17 which is doped in a P type,N⁺-type source/drain regions 22 and P⁺-type source/drain regions 21 areformed as shown in FIG. 157. Thus, the exemplary conventional dual gateCMOS transistor of a polycide gate structure is completed.

FIG. 158 is a sectional view showing another exemplary conventional dualgate CMOS transistor. Referring to FIG. 158, titanium silicide films 23are formed in a self-aligned manner on source/drain regions 21 and 22and polysilicon films 16 and 17. Such a structure obtained by bringingsurfaces of the polysilicon films 16 and 17 and the source/drain regions21 and 22 for forming gate electrodes into silicide states in aself-aligned manner is called a salicide (self aligned silicide)structure. According to this salicide structure, it is possible tosuppress increase of sheet resistance in the source/drain regions 21 and22, which comes into question when the source/drain regions 21 and 22are formed with shallow junction planes.

FIGS. 159 to 163 are sectional views for illustrating a fabricationprocess for the conventional dual gate CMOS transistor shown in FIG.158. With reference to FIGS. 159 to 163, the fabrication process for thedual gate CMOS transistor shown in FIG. 158 is now described.

First, element isolation oxide film 12 are formed on a main surface of aP-type silicon substrate 11, as shown in FIG. 159. Further, an N well 13and a P well 14 are formed on the main surface of the P-type siliconsubstrate 11 to be adjacent to each other. Oxide films 15 a andpolysilicon films (not shown) are successively formed on the N well 13and the P well 14, and thereafter the polysilicon films are patterned toform patterned polysilicon films 8 as shown in FIG. 160.

Then, side wall oxide films 20 are formed on side surfaces of thepolysilicon films 8, and thereafter a PMOS transistor forming region iscovered with a resist film 25, as shown in FIG. 161. The resist film 25is employed as a mask to ion-implant arsenic into the P well 14 and thepolysilicon film 8 provided thereon. Thereafter the resist film 25 isremoved.

Then, an NMOS transistor forming region is covered with a resist film26, and this resist film 26 is employed as a mask to implant boronfluoride ions into the N well 13 and the polysilicon film 8 providedthereon, as shown in FIG. 162. Thereafter the resist film 26 is removed.A titanium layer (not shown) is formed on the overall surface bysputtering, and thereafter heat treatment is carried out to reactsilicon with titanium. Thus, titanium silicide films 23 are formed onsource/drain regions 21 and 22 and polysilicon films 16 and 17, as shownin FIG. 163. Thus, the conventional dual gate CMOS transistor shown inFIG. 158 is completed.

As hereinabove described, the gate electrodes are brought into polycidegate structures of polysilicon films and tungsten silicide films, orbrought into silicide states in the conventional dual gate CMOStransistor, in order to electrically connect the polysilicon film 16which is doped in a P type with the polysilicon film 17 which is dopedin an N type. In particular, it is possible to relieve the source/drainregions from increase of sheet resistance by employing a salicidestructure for the dual gate CMOS transistor.

A thin film transistor (hereinafter referred to as “TFT”) employing apolysilicon film is known as one of semiconductor devices. This TFT isan important device as a load transistor for a highly integrated SRAM ora driving transistor for a liquid crystal display. In relation torequirement for further improvement in integration and performance of anapplied element of such a TFT, however, the TFT itself must be refinedin structure and improved in electrical property as well as reliability.

Important subjects for refinement of the TFT are suppression of a shortchannel effect which is caused by impurity ions, forming source/drainregions, diffused into a channel region, and improvement of hot carrierresistance.

FIG. 164 is a sectional view showing a conventional PMOS-TFT. Referringto FIG. 164, an insulating film 102 is formed on a semiconductorsubstrate 101 in the conventional PMOS-TFT. A gate electrode 103 whichis doped in a P type is formed on the insulating film 102. A gateinsulating film 104 is formed to cover the gate electrode 103. Apolysilicon layer 105 is formed on the gate insulating film 104. AP-type source region 105 b and a P-type drain region 105 c are formed inthe polysilicon layer 105 at a prescribed space, to hold a channelregion 105 a therebetween. FIG. 165 is a perspective view showing anupper portion of the TFT shown in FIG. 164 including the gate electrode103.

FIGS. 166 to 169 are sectional and perspective views for illustrating afabrication process for the TFT shown in FIG. 164. With reference toFIGS. 166 to 169, the fabrication process for the conventional TFT isnow described.

First, an insulating film 102 of a high temperature oxide film is formedon a semiconductor substrate 101 by CVD or the like, as shown in FIG.166. A non-doped polysilicon layer 103 a is formed on the insulatingfilm 102 by CVD or the like. P-type impurity ions such as boron ions,for example, are ion-implanted into the nondoped polysilicon layer 103a. Thereafter a resist film 107 shown in FIG. 167 is formed on aprescribed region of the polysilicon layer 103 a, and this resist film107 is employed as a mask to anisotropically etch the polysilicon layer103 a (see FIG. 166). Thus, a gate electrode 103 b is formed. Thereafterthe resist film 107 is removed. A gate insulating film (not shown) isformed by thermal oxidation, and a non-doped polysilicon layer (notshown) is formed on the gate insulating film by CVD or the like.Thereafter arsenic ions for adjusting a threshold voltage are implantedinto the non-doped polysilicon layer. A resist film (not shown) isformed on a prescribed region of the non-doped polysilicon layer, andthereafter the non-doped polysilicon layer and the gate insulating filmare patterned. Thus, a gate insulating film 104 and a polysilicon layer105 patterned as shown in FIG. 168 are formed. Thereafter the resistfilm is removed.

Then, a resist film 108 is formed on a region of the polysilicon layer105 for forming a channel region. The resist film 108 is employed as amask to ion-implant BF₂ ⁺ into the polysilicon layer 105. Heat treatmentis carried out for activating the impurity as implanted. Thus, a gateelectrode 103, a source region 105 b and a drain region 105 c areformed. Thus, the conventional TFT shown in FIG. 164 is completed.

Another exemplary semiconductor device is a nonvolatile semiconductormemory device. In relation to such a nonvolatile semiconductor memorydevice, known is an EEPROM (electrically erasable and programmable readonly memory) which can freely program data as well as electrically writeand erase data. While this EEPROM can advantageously electrically writeand erase data, it is difficult to highly integrate this memory sincetwo transistors are required for a memory cell. To this end, there hasbeen proposed a flash EEPROM having a memory cell formed by a singletransistor, which can batch-erase written information charges. Such aflash EEPROM is disclosed in U.S. Pat. No. 4,868,619, for example.

FIG. 170 is a sectional view showing a conventional stack gate typeflash EEPROM. Referring to FIG. 170, a drain region 208 and a sourceregion 209 are formed on a main surface of a P-type silicon substrate201 at a prescribed space, to hold a channel region 215 therebetween. Afloating gate electrode 203 is formed on the channel region 215 througha thin oxide film 202 having a thickness of about 100 Å. A control gateelectrode 205 is formed on the floating gate electrode 203 through aninterlayer insulating film 204. The floating gate electrode 203 and thecontrol gate electrode 205 are formed by polysilicon layers. A thermaloxide film 216 is formed to cover the floating gate electrode 203, thecontrol gate electrode 205 and the silicon substrate 201. A smoothcoating film 212 of an oxide film or the like is formed on the thermaloxide film 216. Further, a wiring layer 214 of an aluminum alloy or thelike is formed to cover the smooth coating film 212.

FIG. 171 is a schematic diagram for illustrating a conventional writeoperation of the flash EEPROM employing CHE (channel hot electrons).Referring to FIG. 171, a voltage V_(B1) of 6 to 8 V is applied to thedrain region 208, and a voltage V_(G1) of 10 to 15 V is applied to thecontrol electrode 205. Due to such application of the voltages V_(B1)and V_(G1), electrons having high energy are generated in the vicinityof the drain region 208 and the oxide film 202. Parts of the electronsare attracted to the gate electrode 203 by an electric field which iscaused by the voltage V_(G1) applied to the control gate electrode 205,and injected into the floating gate electrode 203. When the electronsare thus stored in the floating gate electrode 203, a threshold voltageV_(TH) of a control gate transistor exceeds a prescribed value. Thisstate is a written state, which is called a state “0”.

FIG. 172 is a schematic diagram for illustrating a conventional writeoperation of a flash EEPROM employing SHE (substrate hot electrons).With reference to FIG. 172, the write operation employing SHE is nowdescribed. In the flash EEPROM shown in FIG. 172, an N-channel controlgate transistor is formed in a P well 222 which is provided on an N-typesilicon substrate 221. In this case, a drain region 208 and a sourceregion 209 are grounded, and a voltage V_(G2) of 10 to 15 V is appliedto a control gate electrode 205. Further, a voltage V_(B2) of −5 to −10V is applied to a substrate electrode 223. Due to such application ofthe voltages V_(G2) and V_(B2), a P-N junction which is formed by theN-type silicon substrate 221 and the P well 222 is biased in the forwarddirection. Thus, an ON-state current is generated. Parts of electronsforming the ON-state current are attracted to a floating gate electrode203 by an electric field which is caused by the voltage V_(G2) appliedto the control gate electrode 205, and injected into the floating gateelectrode 203.

FIG. 173 is a schematic diagram for illustrating a write operation of aflash EEPROM employing an F-N (Fowler-Nordheim) tunnel phenomenon. Withreference to FIG. 173, the write operation employing the F-N tunnelphenomenon is described. In F-N writing on a drain end, for example, avoltage V_(D3) of −10 to −12 V is applied to a drain region 208.Further, a control gate electrode 205 is held at a ground potential, anda source region 209 is maintained in a floating state. Due to anelectric field caused by the voltage V_(D3) which is applied to thedrain region 208, electrons pass through a thin oxide film 202 by an F-Ntunnel phenomenon, to be injected into a floating gate electrode 203.Thus, the electrons are stored in the floating gate electrode 203,whereby a threshold voltage V_(TH) of a control gate transistor isincreased.

An erase operation is now described. A voltage V_(S) of 10 to 12 V isapplied to the source region 209, while the control gate electrode 205is held at the ground potential and the drain region 208 is maintainedin a floating state. Due to an electric field caused by the voltageV_(S) which is applied to the source electrode 209, the electrons storedin the floating gate electrode 203 pass through the thin oxide film 202by an F-N tunnel phenomenon. Thus, the electrons are extracted from thefloating gate electrode 203, whereby the threshold voltage V_(TH) of thecontrol gate transistor is reduced. Data are erased when the thresholdvoltage V_(TH) is reduced below a prescribed value. This state is calleda state “1”.

In read operation, further, a voltage V_(G4) of 5 V is applied to thecontrol gate electrode 205 and a voltage V_(D4) of 1 to 2 V is appliedto the drain region 208. Determination of the aforementioned state “0”or “1” is made depending on whether or not a current flows in a channelregion of the control gate transistor, i.e., whether the control gatetransistor is in an ON or OFF state. Thus, information is read out.

FIG. 174 is a model diagram for illustrating a coupling ratio of aconventional flash EEPROM. Referring to FIG. 174, the conventional flashEEPROM has a gate electrode of a two-layer structure, whereby a voltagewhich is applied to a control gate electrode 205 is applied to a channelregion through a floating gate electrode 203. In other words, thepotential of the floating gate electrode 203 is varied with thestructures of an interlayer insulating film 204 and an oxide film 202,regardless of the amount of charges stored in the floating gateelectrode 203 and values of potentials applied to respective terminals.A potential V_(FG) of the floating gate electrode 203 depends on athreshold voltage V_(TH), a capacitance C_(FC) between the floating gateelectrode 203 and a control gate electrode 205, a capacitance C_(FB)between the floating gate electrode 203 and a substrate 201, acapacitance C_(FS) between the floating gate electrode 203 and a sourceregion 209, and a capacitance C_(FD) between the floating gate electrode203 and a drain region 208, in addition to potentials applied to therespective terminals such as a control gate voltage V_(CG), a sourcevoltage Vs and a drain voltage V_(D). The potential V_(FG) of thefloating gate electrode 203 is approximately supplied by the followingequation (1):

 V _(FG) =C _(FC) V _(CG) /C _(TOTAL) +C _(FD) V _(D) /C _(TOTAL)+(C_(FD) +C _(FB))V _(S) /C _(TOTAL) +C _(FB) V _(TH) /C _(TOTAL+Q) _(FG)/C _(TOTAL)  (1)

Q _(FG) =C _(FC)(V _(FG) −V _(CG))+C _(FD)(V _(FG) −V _(D))+C _(FS)(V_(FG) −V _(S))+C _(FB)(V _(FG) −V _(TH) −V _(S))

where C_(TOTAL)=C_(FC)+C_(FD)+C_(FS)+C_(FB)

Referring to the above equation (1), the potential V_(CG) of the controlgate electrode 205 exerts an influence on the potential V_(FG) of thefloating gate electrode 203 in multiplication by C_(FC)/C_(TOTAL), whichis called a coupling ratio. When the coupling ratio is large, therefore,the potential V_(CG) of the floating gate electrode 203 is increasedregardless of the potential which is applied to the control gateelectrode 205. Therefore, the transistor operation can be readilycontrolled by the potential which is applied to the control gateelectrode 205, as the coupling ratio is increased.

When data are written and erased through F-N tunnel phenomenons in theaforementioned flash EEPROM, the oxide film 202 is broken in a certainprobability, and hence element reliability is disadvantageously reduced.Due to tunneling of electrons through the oxide film 202, further, theelectrons injected into the oxide film 202 are trapped therein in acertain probability. Thus, an interfacial level is formed in theinterface between the silicon substrate 201 and the oxide film 202. Dueto the interfacial level as formed, reliability of the oxide film 202 isso reduced that the threshold voltage is changed or current drivabilityis reduced as the result. Since a high potential is applied to thefloating gate electrode 203, further, the source region 209 or the drainregion 208 in data writing or erasing, a high electric field is causedin the interface between the drain region 208 or the source region 209and the oxide film 202. In particular, adjacent memory cells share thedrain region 208 in common, and hence a potential is applied also to thedrain region 208 of a nonselected cell in data writing. Since thecontrol gate electrode 205 of the nonselected cell is held at the groundpotential, a high electric field is caused between the floating gateelectrode 203 and the drain region 208. Interband tunneling is caused bythe high electric field as shown in FIG. 175, leading to generation ofelectron-hole pairs. The holes as generated are injected into the oxidefilm 202 in a certain probability, resulting in an interfacial levelwhich is caused in the interface between the silicon substrate 201 andthe oxide film 202. Thus, the oxide film 202 is reduced in reliability.

In order to prevent such reduction in reliability of the oxide film 202,there has been proposed a method of suppressing generation of aninterfacial level in the interface between the silicon substrate 201 andthe oxide film 202. For example, there has been proposed a method ofcarrying out RTN (rapid thermal nitridation) treatment after formationof the oxide film 202, to introduce nitrogen into the oxide film 202.Since nitrogen terminates dangling bonds in the oxide film 202, wherebyit is possible to prevent the oxide film 202 from trapping chargestherein. The RTN treatment is adapted to carry out annealing for anextremely short time in a reactive gas atmosphere containing nitrogensuch as ammonia (NH3), for example. Thus, nitrogen is incorporated intothe silicon substrate 201 and the oxide film 202.

FIG. 176 is a sectional view showing a conventional buried channel typeflash EEPROM. Referring to FIG. 176, an N-type impurity layer 217 isformed on a surface of a channel region 215, and a P-type impurity layer218 is formed under the N-type impurity layer 217 in this buried channeltype flash EEPROM. A buried channel layer is formed by the N-type andP-type impurity layers 217 and 218. In such a buried channel type flashEEPROM, no high electric field is applied across a source region 209 ora drain region 208 and an oxide film 202 dissimilarly to a surfacechannel type flash EEPROM, whereby it is possible to suppress occurrenceof interband tunneling in this region. Therefore, it is possible toprevent generation of holes caused by interband tunneling in datawriting or erasing, thereby preventing the oxide film 202 from injectionof holes.

However, the conventional MOS transistors have the following problems:

In the conventional method of forming source/drain regions of a PMOStransistor shown in FIG. 145, boron fluoride ions having a large massare implanted for forming the source/drain regions 6 with shallowjunction planes. However, fluorine contained in the boron fluoride ionsdisadvantageously hinders reaction between titanium and silicon information of titanium silicide. Thus, excellent titanium silicide filmscannot be formed on the surfaces of the source/drain regions 6 and thegate electrode 3.

In the conventional method of forming source/drain regions of a PMOStransistor shown in FIGS. 146A and 146B, the surface of the N-typesilicon substrate 1 is brought into a preamorphous state by implantationof silicon ions or germanium ions, and hence high temperature heattreatment is required for crystal recovery. However, it is necessary toreduce the heat treatment in order to form the source/drain regions 6with shallow junction planes, and hence crystal recovery is renderedinsufficient, leading to increase of a junction leakage current. Thisproblem is similarly caused also in formation of source/drain regions ofan NMOS transistor.

In the conventional method of forming source/drain regions, further, itis difficult to form source/drain regions having shallow junction planessince impurities as implanted are diffused by heat treatment foractivation in both of PMOS and NMOS transistors.

In the conventional NMOS and PMOS transistors, further, impurities dopedin the gate electrodes are diffused in heat treatment steps todeteriorate the gate oxide films. Consequently, no sufficient hotcarrier resistance can be attained upon progress of element refinement.

In each of the dual gate CMOS transistors shown in FIGS. 148 and 158,boron ions disadvantageously enter the channel region from the gateelectrode of the PMOS transistor which is doped in a P type through thegate oxide film in the heat treatment step. Therefore, the thresholdvoltage of the transistor is disadvantageously changed. Particularly inthe dual gate CMOS transistor of the polycide gate structure, arsenicions and boron ions are mutually diffused in the silicide from the gateelectrodes which are doped in N and P types respectively during the heattreatment step. Thus, work functions of the gate electrodes are varied,leading to fluctuation of the threshold voltages of the transistors.

In the conventional TFT shown in FIG. 164, on the other hand, thefollowing problems are caused along progress of refinement. Namely, theimpurity which is ion-implanted for forming the source/drain regions 105b and 105 c is thermally diffused by the later heat treatment anddisadvantageously diffused toward the channel region 105 a. Thus, apunch-through phenomenon is caused to inhibit an original transistoroperation. The punch-through phenomenon is such a phenomenon that adepletion layer in the vicinity of a drain is spread toward a sourceregion when a channel length is small, and hence no current can becontrolled by a gate voltage.

In the conventional TFT, further, hot carriers are generated when theelectric field which is applied to the drain region 105 c is increasedin an OFF state, resulting in deterioration of element reliability.

On the other hand, the conventional flash EEPROM shown in FIG. 170 hasthe following problems: In general, RTN treatment is employed as amethod of introducing nitrogen into the oxide film 202. However, the RTNtreatment is generally adapted to carry out annealing in an ammoniaatmosphere, and hence not only nitrogen but hydrogen is introduced intothe oxide film 202, as shown in FIG. 177. The oxide film 202 isdisadvantageously reduced in reliability due to such doping withhydrogen. Further, hydrogen and nitrogen are disadvantageously injectedalso into the silicon substrate 201 on the fabrication process.

In the RTN treatment, further, the silicon substrate 201 is exposed to ahigh temperature of about 1100° C. while the treatment is carried out ina short time, and hence the peripheral temperature to which the siliconsubstrate 201 is exposed is abruptly changed. Thus, temperaturedistribution is caused in a prescribed plane of the silicon substrate201, to cause slit-shaped defects due to difference in coefficient ofexpansion.

Further, the potential which is applied to the control gate electrode205 is applied to the floating gate electrode 203 in multiplication bythe coupling ratio. Therefore, it is necessary to apply the potential tothe control gate electrode 205 in consideration of reduction by thecoupling ratio. In order to apply a voltage of 5 V to the floating gateelectrode 203 for writing data in a device having a coupling ratio of0.5, for example, it is necessary to apply a voltage of about 10 V tothe control gate electrode 205. Namely, the voltage which is applied tothe control electrode 205 must be increased as the coupling ratio isreduced in order to guarantee a stable operation, and hence it isdifficult to reduce the voltage of a power source for the flash EEPROM.

In general, there has been proposed a method of employing preparing theinterlayer insulating film 204 from a nitride film having a higherdielectric constant than an oxide film, in order to improve the couplingratio. When the interlayer insulating film 204 is formed only by anitride film, however, a leakage current is disadvantageously increased.When the interlayer insulating film 204 is prepared from a compositefilm of a nitride film and an oxide film in order to prevent the problemof a leakage current, the interlayer insulating film 204 isdisadvantageously increased in thickness. Thus, it is impossible toincrease the coupling ratio.

In the conventional buried channel type flash EEPROM shown in FIG. 176,it is difficult to form a shallow buried channel layer by diffusion ofan impurity which is implanted into the buried channel region. Thus, itis impossible to control the current between the source region 209 andthe drain region 208 by the potential which is applied to the controlgate electrode 205, and hence inconvenience such as a punch-throughphenomenon is caused as the result.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor deviceincluding source/drain regions having shallow junction planes.

Another object of the present invention is to provide a semiconductordevice which can prevent diffusion of an impurity doped into a gateelectrode.

Still another object of the present invention is to provide asemiconductor device (nonvolatile semiconductor memory device) having alarge coupling ratio.

A further object of the present invention is to provide a semiconductordevice (nonvolatile semiconductor memory device) which can effectivelyprevent interband tunneling, as well as improve an oxide film and aninterlayer insulating film in reliability.

A further object of the present invention is to provide a method offabricating a semiconductor device which can easily form source/drainregions having shallow junction planes.

A further object of the present invention is to provide a method offabricating a semiconductor device which can prevent diffusion of animpurity in a gate electrode caused by thermal diffusion.

In a first aspect of the present invention, a semiconductor devicecomprises a semiconductor substrate, source/drain regions, a firstinsulating film, and a first electrode. The source/drain regions areformed on a main surface of the semiconductor substrate at a prescribedspace, to define a channel region therebetween. The first insulatingfilm is formed on the channel region. The first electrode, which isprovided on the first insulating film, has a first conductivity type.The first insulating film and the first electrode are doped withnitrogen.

In this semiconductor device, the first electrode (gate electrode) isdoped with nitrogen, whereby nitrogen previously occupies holes whichare diffusion paths for an impurity when the impurity is introduced intothe gate electrode. Thus, the impurity is inhibited from diffusion.Consequently, the impurity is inhibited from being mixed into the gateinsulating film (first insulating film) or passing through the gateinsulating film. Further, nitrogen is deposited in the gate insulatingfilm, thereby suppressing occurrence of an interfacial level in theinterface between the gate insulating film and the semiconductorsubstrate. Thus, the gate insulating film of the semiconductor device isimproved in reliability and hot carrier resistance. When thesemiconductor device is an electrically erasable and programmablesemiconductor memory device, further, it is possible to preventoccurrence of an interfacial level in the interface between the gateinsulating film and the semiconductor substrate caused by interbandtunneling or an F-N tunnel phenomenon, since nitrogen is deposited inthe first insulating film provided under the floating gate electrode.

In a method of fabricating a semiconductor device according to a secondaspect of the present invention, an insulating film is formed on asemiconductor substrate. An electrode layer is formed on the insulatingfilm. Nitrogen is ion-implanted into the electrode layer. Further, animpurity is ion-implanted into the electrode layer. Heat treatment iscarried out after nitrogen and the impurity are implanted into theelectrode layer, so that nitrogen is deposited in the insulating film.

According to this method of fabricating a semiconductor device, heattreatment is carried out after nitrogen is ion-implanted into a gateelectrode (electrode layer) which is formed on a gate insulating film(insulating film) so that nitrogen is deposited in the gate insulatingfilm, whereby nitrogen is implanted into the gate insulating filmwithout damaging the same. Further, no hydrogen is introduced into thegate insulating film. Thus, an effect of nitrogen introduction isattained with no bad influence exerted by hydrogen.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a PMOS transistor according to afirst embodiment of the present invention;

FIG. 2 illustrates an impurity profile in a depth direction of a gateelectrode of the PMOS transistor shown in FIG. 1;

FIGS. 3 to 7 are sectional views for illustrating a fabrication processfor the PMOS transistor according to the first embodiment shown in FIG.1;

FIG. 8 is a graph for illustrating nitrogen implantation conditions in astep of fabricating the PMOS transistor according to the firstembodiment;

FIG. 9 is a graph for illustrating improvement in reliability of anoxide film by nitrogen implantation;

FIG. 10 is a graph for illustrating dependence of change of a thresholdvoltage by hot carrier injection on an injection rate of nitrogen in thePMOS transistor;

FIG. 11 is a sectional view showing a PMOS transistor according to asecond embodiment of the present invention;

FIG. 12 illustrates an impurity profile in a depth direction of asource/drain region of the PMOS transistor according to the secondembodiment shown in FIG. 11;

FIGS. 13 to 17 are sectional views for illustrating an exemplaryfabrication process for the PMOS transistor according to the secondembodiment shown in FIG. 11;

FIGS. 18 and 19 are sectional views for illustrating another exemplaryfabrication process for the PMOS transistor according to the secondembodiment shown in FIG. 11;

FIG. 20 is a sectional view showing a modification of the PMOStransistor according to the second embodiment shown in FIG. 11;

FIG. 21 illustrates an impurity profile along a depth direction of thesource/drain region of the PMOS transistor according to the secondembodiment shown in FIG. 11;

FIG. 22 is a sectional view showing a PMOS transistor according to athird embodiment of the present invention;

FIGS. 23 to 26 are sectional views for illustrating an exemplaryfabrication process for the PMOS transistor according to the thirdembodiment shown in FIG. 22;

FIGS. 27 to 32 are sectional views for illustrating another exemplaryfabrication process for the PMOS transistor according to the thirdembodiment shown in FIG. 22;

FIG. 33 is a sectional view for illustrating a modification of the PMOStransistor according to the third embodiment shown in FIG. 22;

FIG. 34 is a sectional view showing an NMOS transistor according to afourth embodiment of the present invention;

FIG. 35 illustrates an impurity profile along a depth direction of agate electrode and a gate oxide film of the NMOS transistor according tothe fourth embodiment shown in FIG. 34;

FIGS. 36 to 41 are sectional views for illustrating a fabricationprocess for the NMOS transistor according to the fourth embodiment shownin FIG. 34;

FIG. 42 is a graph showing relation between of an injection rate ofnitrogen with respect to a gate electrode and a threshold voltage;

FIG. 43 is a sectional view showing an NMOS transistor according to afifth embodiment of the present invention;

FIG. 44 illustrates an impurity profile in a depth direction of an N⁺source/drain region of the NMOS transistor shown in FIG. 43;

FIGS. 45 to 48 are sectional views for illustrating a fabricationprocess for the NMOS transistor according to the fifth embodiment shownin FIG. 43;

FIG. 49 is a sectional view for illustrating a modification of the NMOStransistor according to the fifth embodiment shown in FIG. 43;

FIG. 50 is a sectional view showing a dual gate CMOS transistoraccording to a sixth embodiment of the present invention;

FIGS. 51 to 59 are sectional views for illustrating an exemplaryfabrication process for the dual gate CMOS transistor according to thesixth embodiment shown in FIG. 50;

FIGS. 60 to 64 are sectional views for illustrating another exemplaryfabrication process for the dual gate CMOS transistor according to thesixth embodiment shown in FIG. 50;

FIG. 65 is a sectional view showing a dual gate CMOS transistoraccording to a seventh embodiment of the present invention;

FIG. 66 illustrates an impurity profile along a depth direction of agate electrode of a PMOS transistor provided in the seventh embodimentshown in FIG. 65;

FIG. 67 illustrates an impurity profile in a depth direction of a gateelectrode of an NMOS transistor provided in the seventh embodiment shownin FIG. 65;

FIGS. 68 to 76 are sectional views for illustrating a fabricationprocess for the dual gate CMOS transistor according to the seventhembodiment shown in FIG. 65;

FIG. 77 is a sectional view showing a PMOS-TFT according to an eighthembodiment of the present invention;

FIG. 78 illustrates an impurity profile along a depth direction of theline a—a in the PMOS-TFT according to the eighth embodiment shown inFIG. 77;

FIG. 79 illustrates an impurity profile along a depth direction of theline b—b in the PMOS-TFT according to the eighth embodiment shown inFIG. 77;

FIG. 80 is a perspective view for illustrating a fabrication process forthe PMOS-TFT according to the eighth embodiment shown in FIG. 77:

FIGS. 81 and 82 are sectional views for illustrating the fabricationprocess for the PMOS-TFT according to the eighth embodiment shown inFIG. 77:

FIG. 83 illustrates an impurity profile along a depth direction of theline a—a (see FIG. 77) in an NMOS-TFT according to a ninth embodiment ofthe present invention;

FIG. 84 illustrates an impurity profile along a depth direction of theline b—b (see FIG. 77) in the NMOS-TFT according to the ninthembodiment;

FIG. 85 is a sectional view showing a PMOS-TFT according to a tenthembodiment of the present invention;

FIG. 86 illustrates an impurity profile along a depth direction of theline a—a in the PMOS-TFT shown in FIG. 85;

FIGS. 87 to 89 are sectional and perspective views for illustrating afabrication process for the PMOS-TFT according to the tenth embodimentshown in FIG. 85;

FIG. 90 illustrates an impurity profile along a depth direction of theline a—a (see FIG. 85) in an NMOS-TFT according to an eleventhembodiment of the present invention;

FIG. 91 is a sectional view showing a PMOS-TFT according to a twelfthembodiment of the present invention;

FIG. 92 illustrates an impurity profile along a depth direction of theline a—a in the PMOS-TFT according to the twelfth embodiment shown inFIG. 91;

FIGS. 93 to 96 are sectional and perspective views for illustrating afabrication process for the PMOS-TFT according to the twelfth embodimentshown in FIG. 91;

FIG. 97 illustrates an impurity profile along a depth direction of theline a—a (see FIG. 91) in an NMOS-TFT according to a thirteenthembodiment of the present invention;

FIG. 98 is a perspective view showing a dual gate CMOS-TFT according toa fourteenth embodiment of the present invention;

FIG. 99 is a sectional view of the dual gate CMOS-TFT shown in FIG. 98taken along the line 300 to 300;

FIG. 100 is a sectional view of the dual gate CMOS-TFT shown in FIG. 98taken along the line 400—400;

FIG. 101 illustrates an impurity profile along a depth direction of theline b—b in the dual gate CMOSTFT shown in FIG. 99;

FIG. 102 illustrates an impurity profile in a depth direction of theline b—b in the dual gate CMOS-TFT shown in FIG. 99;

FIGS. 103 to 108 are sectional and plan views for illustrating afabrication process for the dual gate CMOS-TFT according to thefourteenth embodiment shown in FIG. 98;

FIG. 109 is a sectional view showing a stack gate type flash EEPROMaccording to a fifteenth embodiment of the present invention;

FIG. 110 illustrates a nitrogen concentration profile in a depthdirection of a control gate electrode, an interlayer insulating film, afloating gate electrode and an oxide film in the flash EEPROM shown inFIG. 109;

FIGS. 111, 112 and 114 to 124 are sectional views for illustrating afabrication process for the stack gate type flash EEPROM according tothe fifteenth embodiment shown in FIG. 109;

FIG. 113 is a graph for illustrating a method of setting a nitrogenprojection range in a nitrogen implantation step shown in FIG. 112;

FIG. 125 is a sectional view showing a memory cell part of a stack gatetype flash EEPROM according to a sixteenth embodiment of the presentinvention;

FIGS. 126 to 128 are sectional views for illustrating a fabricationprocess for the stack gate type flash EEPROM according to the sixteenthembodiment shown in FIG. 125;

FIG. 129 is a sectional view showing a memory cell part of a stack gatetype flash EEPROM according to a seventeenth embodiment of the presentinvention;

FIGS. 130 and 131 are sectional views for illustrating a fabricationprocess for the stack gate type flash EEPROM according to theseventeenth embodiment shown in FIG. 129;

FIG. 132 is a sectional view showing a memory cell part of a buriedchannel type flash EEPROM according to an eighteenth embodiment of thepresent invention;

FIGS. 133 to 136 are sectional views for illustrating a fabricationprocess for the buried channel type flash EEPROM according to theeighteenth embodiment shown in FIG. 132;

FIG. 137 is a sectional view showing a stack gate type flash EEPROMaccording to a nineteenth embodiment of the present invention;

FIG. 138 illustrates an impurity profile in a depth direction of a drainregion in the flash EEPROM shown in FIG. 137;

FIGS. 139 to 141 are sectional views for illustrating a fabricationprocess for the stack gate type flash EEPROM according to the nineteenthembodiment shown in FIG. 137;

FIG. 142 is a sectional view showing a stack gate type flash EEPROMaccording to a twentieth embodiment of the present invention;

FIG. 143 is a sectional view showing a stack gate type flash EEPROMaccording to a twenty-first embodiment of the present invention;

FIG. 144 is a sectional view for illustrating a fabrication process forthe stack gate type flash EEPROM according to the twenty-firstembodiment shown in FIG. 143;

FIG. 145 is a sectional view for illustrating a first example of aconventional method of fabricating a PMOS transistor;

FIGS. 146A and 146B are sectional views for illustrating a secondexample of a conventional method of fabricating a PMOS transistor;

FIG. 147 is a sectional view showing a conventional improved PMOStransistor;

FIG. 148 is a sectional view showing an example of a conventional dualgate CMOS transistor;

FIGS. 149 to 157 are sectional views for illustrating a fabricationprocess for the dual gate CMOS transistor shown in FIG. 148;

FIG. 158 is a sectional view showing another example of a conventionaldual gate CMOS transistor;

FIGS. 159 to 163 are sectional views for illustrating a fabricationprocess for the dual gate CMOS transistor shown in FIG. 158;

FIG. 164 is a sectional view showing a conventional PMOS-TFT;

FIG. 165 is a perspective view of the PMOS-TFT shown in FIG. 164;

FIGS. 166 to 169 are sectional and perspective views for illustrating afabrication process for the conventional PMOS-TFT shown in FIG. 164;

FIG. 170 is a sectional view showing a conventional stack gate typeflash EEPROM;

FIG. 171 is a schematic diagram for illustrating a write operation inthe conventional flash EEPROM employing CHE;

FIG. 172 is a schematic diagram for illustrating a write operation in aconventional flash EEPROM employing SHE;

FIG. 173 is a schematic diagram for illustrating a write operation in aconventional flash EEPROM employing an F-N tunnel phenomenon;

FIG. 174 is a model diagram for illustrating a coupling ratio of a flashEEPROM;

FIG. 175 is a model diagram for illustrating interband tunneling;

FIG. 176 is a sectional view showing a conventional buried channel typeflash EEPROM; and

FIG. 177 is a model diagram for illustrating inconvenience in a case ofemploying RTN treatment as a method of introducing nitrogen into anoxide film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are now described with reference tothe drawings.

Referring to FIG. 1, element isolation oxide film 7 are formed on a mainsurface of an N-type silicon substrate 1 in a PMOS transistor accordingto a first embodiment of the present invention. Source/drain regions 6are formed on a portion of the main surface of the N-type siliconsubstrate 1 which is held between the element isolation oxide film 7 ata prescribed space, to define a channel region 10 therebetween. AP⁺-type gate electrode 35 is formed on the channel region 10 through agate oxide film 36. An oxide film 4 is formed on the P⁺-type gateelectrode 35. Side wall oxide films 5 are formed on side surfaces of theP⁺-type gate electrode 35 and the oxide film 4.

In the PMOS transistor according to the first embodiment, anitrogen-doped region 30 shown by slant lines is doped with nitrogen.Namely, nitrogen is introduced into the P⁺-type gate electrode 35 andthe gate oxide film 36. Referring to FIG. 2, it is understood thatnitrogen (N) is deposited in the gate oxide film 36. The wording“deposition of nitrogen” indicates such a state that nitrogen is trappedin a certain constant position to be increased in concentration.

With reference to FIGS. 3 to 7, a method of fabricating the PMOStransistor according to the first embodiment shown in FIG. 1 is nowdescribed.

First, element isolation oxide film 7 are formed on an N-type substrate1 by an ordinary element isolation step, as shown in FIG. 3. Thereafteran oxide film 36 a of about 100 Å in thickness is formed on the overallsurface by thermal oxidation. A polysilicon film 35 a of about 200 Å inthickness is formed on the overall oxide film 36 a by CVD.

Then, nitrogen ions are implanted into the polysilicon film 35 a underconditions of 20 keV and 4×10¹⁵/cm², so that a range center of thenitrogen ions comes to an upper portion of the polysilicon film 35 a, asshown in FIG. 4. Thereafter boron ions are implanted into thepolysilicon film 35 a under conditions of 20 keV and 4×10¹⁵/cm², asshown in FIG. 5. An oxide film (not shown) of about 2000 Å in thicknessis formed by CDV, and this oxide film and the polysilicon film 35 a arepatterned by photolithography and dry etching. Thus, a gate electrode 35b and an oxide film 4 are formed as shown in FIG. 6.

An oxide film (not shown) of about 800 Å in thickness is formed by CDV,and this oxide film is thereafter etched back to form side wall oxidefilms 5 and a gate oxide film 36 b as shown in FIG. 7. The side walloxide films 5 and the element isolation oxide film 7 are employed asmasks to implant boron fluoride ions into the N-type silicon substrate 1under conditions of 20 keV and 4×10¹⁵/cm². Heat treatment is carried outat 850° C. for about 20 minutes, thereby activating the impurities asimplanted. Thus, the source/drain regions 6 and the P⁺-type gateelectrode 35 are formed as shown in FIG. 1. In this heat treatment,nitrogen which is doped into an upper portion of the gate electrode 35 bis thermally diffused, while nitrogen segregates in the gate oxide film36 b. Thus, a concentration peak of nitrogen exists in the gate oxidefilm 36, as shown in FIG. 2.

The nitrogen implantation conditions in the aforementioned fabricationprocess are now described in further detail with reference to FIG. 8. Aprojection range R_(P) of nitrogen is set to come to a position of theP⁺-type gate electrode 35 upward beyond a position separated by 5×ΔR_(P)from the interface between the P⁺-type gate electrode 35 and the gateoxide film 36 toward the P⁺-type gate electrode 35, assuming that ΔR_(P)represents its standard deviation. If the projection range R_(p) is setdownward beyond the said position, the gate oxide film 36 may be damagedby nitrogen implantation.

While the P⁺-type gate electrode 35 and the source/drain regions 6 aredoped through different steps in the aforementioned fabrication process,the P⁺-type gate electrode 35 may alternatively be doped with nitrogensimultaneously with the source/drain regions 6. Further, the P⁺-typegate electrode 35 may alternatively be doped by implantation of boronfluoride ions. While the first embodiment has been described withreference to a PMOS transistor alone, this embodiment may also beapplied to a PMOS transistor which is included in a CMOS transistor.

Effects of the aforementioned first embodiment are now described. TheP⁺-type gate electrode 35 is doped with nitrogen, whereby diffusion ofboron is suppressed. Namely, nitrogen is in the same hole diffusionmechanism as boron, and has a larger diffusion coefficient than boron.When nitrogen is mutually diffused with boron, therefore, nitrogenoccupies holes, which are diffusion paths, in advance. Consequently, itis possible to suppress diffusion of boron, thereby inhibiting boronfrom punching through the channel region 10. Thus, it is possible toeffectively suppress fluctuation of the threshold voltage. Further, thedepth and concentration distribution of nitrogen as doped can be readilycontrolled through ion implantation.

The P⁺-type gate electrode 35 is doped with nitrogen in its upperportion and heat treated, whereby nitrogen is deposited in the gateoxide film 36. Consequently, an interfacial level is reduced in theinterface between the gate oxide film 36 which is formed by a siliconoxide film and the N-type silicon substrate 1 which is made of silicon.Thus, it is possible to improve reliability of the gate oxide film 36,while effectively improving hot carrier resistance.

FIG. 9 shows levels of reliability of gate oxide films provided in aconventional MOS transistor and a MOS transistor having a gate electrodedoped with nitrogen, which were evaluated by a constant current stressmethod. Referring to FIG. 9, it is understood that dielectric breakdownresistance is improved when nitrogen is implanted into the gateelectrode 35 so that nitrogen is deposited in the gate oxide film 36.Thus, the gate oxide film 36 is improved in reliability.

FIG. 10 shows dependence of change in threshold voltage of a PMOStransistor, caused by hot carrier implantation, on a nitrogen injectionrate. Referring to FIG. 10, the change of threshold voltage was measuredafter application of a constant stress voltage for about 1000 seconds.It is understood that the change of the threshold voltage is reducedwhen the nitrogen injection rate for the gate electrode 35 is increased.In other words, it is understood that hot carrier resistance of the PMOStransistor is improved when the gate electrode 35 is doped with nitrogento deposit nitrogen in the gate oxide film 36.

A nitrogen concentration peak in the nitrogen-doped region 30 in thegate electrode 35 and the gate oxide film 36 is preferably set in arange of 10¹⁹/cm³ to 10²¹/cm³. Therefore, the injection rate fornitrogen ions may be set in a range of 10¹⁴/cm² to 10¹⁶/cm² in thefabrication step. The aforementioned effects cannot be attained if thenitrogen concentration peak is lower than 10¹⁹/cm³. When the nitrogenconcentration peak in the gate oxide film 36 is higher than 10²¹/cm³,mobility of channel electrons is deteriorated and hence electricalproperties of the MOS transistor are deteriorated.

With reference to FIG. 11, a PMOS transistor according to a secondembodiment of the present invention is now described. As shown in FIG.11, nitrogen-doped regions 30 are formed in source/drain regions 6according to the second embodiment. Referring to FIG. 12, it isunderstood that junction planes of the source/drain regions 6 shown inFIG. 11 are not doped with nitrogen but contains only boron. Namely, itis understood that the nitrogen-doped regions 30 exist in thesource/drain regions 6 which are doped with boron.

With reference to FIGS. 13 to 17, an exemplary fabrication process forthe PMOS transistor according to the second embodiment shown in FIG. 11is now described.

First, element isolation oxide film 7 are formed on an N-type siliconsubstrate 1, as shown in FIG. 13. Then, an oxide film 2 a of about 100 Åin thickness is formed by thermal oxidation. Thereafter a polysiliconfilm 3 a which is doped with phosphorus by about 5×20/cm³ is formed in athickness of about 2000 Å by CDV. Further, an oxide film 4 a of about2000 Å in thickness is formed on the polysilicon film 3 a by CDV. Theoxide film 4 a and the polysilicon film 3 a are patterned byphotolithography and anisotropic etching, thereby forming a gateelectrode 3 and an oxide film 4 shown in FIG. 14.

An oxide film (not shown) of about 800 Å in thickness is formed on theoverall surface by CDV, and thereafter this oxide film is etched back toform side wall oxide films 5, as shown in FIG. 15.

Then, the side wall oxide films 5 and the element isolation oxide film 7are employed as masks to implant nitrogen into the N-type siliconsubstrate 1 under conditions of 10 keV and 2×10¹⁵/cm², as shown in FIG.16. Thereafter the side wall oxide films 5 and the element isolationoxide film 7 are again employed as masks to ion-implant boron into theN-type silicon substrate 1, as shown in FIG. 17. Then heat treatment iscarried out at 850° C. for about 20 minutes, thereby activating theimpurities as implanted. Thus, the source/drain regions 6 and thenitrogen-doped regions 30 are formed as shown in FIG. 11.

With reference to FIGS. 18 and 19, another exemplary fabrication processfor the PMOS transistor according to the second embodiment shown in FIG.11 is now described.

A process up to formation of side wall oxide films 5 is similar to thatdescribed with reference to FIGS. 13 to 15. Thereafter the side wallfilms 5 and element isolation oxide film 7 are employed as masks toimplant nitrogen ions into an N-type silicon substrate 1 at an angle ofincidence of 30°. In more concrete terms, the N-type silicon substrate 1is rotated so that nitrogen ions are implanted into the same underconditions of 12 keV and 2.5×10¹⁵/cm².

Then, boron ions are implanted into the N-type silicon substrate 1 underconditions of 10 keV and 4×10¹⁵/cm², as shown in FIG. 19. Thereafterheat treatment is carried out at 850° C. for about 20 minutes, therebyactivating the impurities as implanted. Thus, the source/drain regions 6and the nitrogen-doped regions 30 are formed as shown in FIG. 11.

The nitrogen implantation conditions in the fabrication process for thePMOS transistor according to the second embodiment are now described infurther detail. Nitrogen is implanted at such energy that the projectionrange of nitrogen is smaller than that of boron, so that no defects arecaused in the junction planes between the source/drain regions 6 and theN-type silicon substrate 1 in nitrogen implantation and no junctionleakage current is caused in element operation.

While the N-type gate electrode 3 is employed in the above description,the same may be replaced by a P-type gate electrode, or a gate electrodehaving a layered structure of a metal silicide layer and a polysiliconlayer so that the gate electrode is reduced in sheet resistance.Further, titanium silicide films 8 may be formed on the source/drainregions 6 as shown in FIG. 20, in order to reduce the source/drainregions 6 in resistance. These titanium silicide films 8 can be readilyformed by employing a titanium salicide step after the step describedwith reference to FIG. 19. While boron is ion-implanted in formation ofthe source/drain regions 6 in the aforementioned fabrication process forthe PMOS transistor according to the second embodiment, further, boronfluoride ions may alternatively be implanted into the source/drainregions 6 if no titanium silicide films 8 shown in FIG. 20 are formedthereon. While the second embodiment has been described with referenceto a PMOS transistor alone, this embodiment may also be applied to aCMOS transistor including a PMOS transistor.

Effects of the second embodiment are now described. The P⁺-typesource/drain regions 6 are doped with nitrogen, whereby diffusion ofboron is suppressed. Namely, nitrogen is in the same hole diffusionmechanism as boron and has a larger diffusion coefficient than boron,and hence nitrogen and boron are mutually diffused. Thus, nitrogenoccupies holes, which are diffusion paths, in advance, whereby it ispossible to suppress diffusion of boron. Thus, it is possible to formthe source/drain regions 6 with shallow junction planes. In the secondembodiment, nitrogen is implanted at such energy that the projectionrange of nitrogen is smaller than that of boron, in order to preventdamage caused by nitrogen implantation. Also in this case, it ispossible to sufficiently suppress diffusion of boron even if the end ofnitrogen concentration distribution in nitrogen implantation is notdeeper than that of boron concentration distribution in boronimplantation, as shown in FIG. 21.

When nitrogen is doped by ion implantation, further, the N-type siliconsubstrate 1 is brought into an amorphous state, whereby it is possibleto suppress a channeling phenomenon in subsequent ion implantation ofboron. Thus, it is possible to form the source/drain regions 6 withshallow junction planes. Further, the N-type silicon substrate 1 isbrought into an amorphous state by nitrogen at a smaller degree, ascompared with an amorphous state implemented by ion implantation ofgermanium and silicon, whereby no high temperature heat treatment isrequired for crystal recovery. Thus, this method is further effectivefor formation of shallow junction planes. Further, the source/drainregions 6 can be formed with no implantation of boron fluoride, wherebyit is possible to eliminate prevention of a silicide reaction caused byfluorine contained in boron fluoride when the source/drain regions 6 arereduced in resistance through a salicide step. Consequently, it ispossible to form an excellent metal silicide film. Further, it ispossible to further suppress transverse diffusion of boron by dopingnitrogen by oblique rotation implantation. As the result, it is possibleto increase the effective gate length of the transistor.

A nitrogen concentration peak in the nitrogen-doped region 30 in eachsource/drain region 6 is preferably set in a range of 10¹⁹/cm³ to10²¹/cm³. Therefore, the injection rate for nitrogen ions in thefabrication process may be set in a range of 10¹⁴/cm² to 10¹⁶/cm². Noneof the aforementioned effects can be attained if the nitrogenconcentration peak is lower than 10¹⁹/cm³, while the rate of activationof boron is reduced and each source/drain region 6 is increased inresistance if the nitrogen concentration peak is higher than 10²¹/cm³.

With reference to FIG. 22, a PMOS transistor according to a thirdembodiment of the present invention is now described. As shown in FIG.22, the third embodiment has a structure obtained by combining the firstand second embodiments with each other.

With reference to FIGS. 23 to 26, an exemplary fabrication process forthe PMOS transistor according to the third embodiment shown in FIG. 22is now described.

First, element isolation oxide film 7 and an oxide film 36 a of about100 Å in thickness are formed on an N-type silicon substrate 1, as shownin FIG. 23. A polysilicon film 35 a of about 2000 Å in thickness isformed by CDV. The polysilicon film 35 a and the oxide film 36 a arepatterned by photolithography and anisotropic etching. Thus, a patternedpolysilicon film 35 b is obtained as shown in FIG. 24. Thereafter anoxide film (not shown) of about 800 Å in thickness is formed by CVD andthis oxide film is etched back, to form side wall oxide films 5 and agate oxide film 36 b.

Then, nitrogen ions are implanted into the polysilicon film 35 b andsource/drain forming regions under conditions of 10 keV and 2×10¹⁵/cm²,so that a range center comes to an upper portion of the polysilicon film35 b, as shown in FIG. 25.

Then, boron ions are implanted into the polysilicon film 35 b and thesource/drain forming regions under conditions of 10 keV and 4×10¹⁵/cm²,as shown in FIG. 26. Thereafter heat treatment is carried out at 850° C.for about 20 minutes, thereby activating the impurities as implanted.Thus, source/drain regions 6, a gate electrode 35 and nitrogen-dopedregions 30 are formed as shown in FIG. 22. While nitrogen which is dopedinto the upper portion of the gate electrode 35 b is thermally diffusedin the heat treatment, nitrogen segregates in the gate oxide film 36 b.Thus, a gate oxide film 36 having a nitrogen concentration peak as shownin FIG. 2 is formed. The conditions of nitrogen implantation into thesource/drain regions 6 and the gate electrode 35 are as described abovewith reference to the first and second embodiments.

With reference to FIGS. 27 to 32, another exemplary fabrication processfor the PMOS transistor according to the third embodiment shown in FIG.22 is now described.

First, element isolation oxide film 7, an oxide film 36 a of about 100 Åin thickness and a polysilicon film 35 a of about 2000 Å in thicknessare formed on an N-type silicon substrate 1, as shown in FIG. 27.

Then, nitrogen ions are implanted into the polysilicon film 35 a underconditions of 20 keV and 4×10¹⁵/cm², so that a projection range comes toa portion of the polysilicon film 35 a upward beyond its thicknesscenter, as shown in FIG. 28. Thereafter boron ions are further implantedinto the polysilicon film 35 a under conditions of 20 keV and4×10¹⁵/cm². Then, the polysilicon film 35 a is patterned byphotolithography and anisotropic etching, to form a gate electrode 35 bshown in FIG. 30. Thereafter an oxide film (not shown) of about 800 Å inthickness is formed by CVD and this oxide film is etched back, to formside wall oxide films 5 and a gate oxide film 36 b.

Then, nitrogen is ion-implanted into source/drain forming regions underconditions of 10 keV and 2×10¹⁵/cm², as shown in FIG. 31. Thereafterboron ions are implanted into the source/drain forming regions underconditions of 10 keV and 4×10¹⁵/cm², as shown in FIG. 32. Finally, heattreatment is carried out at 850° C. for about 20 minutes.

While nitrogen and boron are implanted into the gate electrode 35 twicein the aforementioned second fabrication process for the PMOS transistoraccording to the third embodiment, the present invention is notrestricted to this but the following fabrication process is alsoemployable: After the step shown in FIG. 29, an oxide film of about 2000Å in thickness is formed on the polysilicon film 35 a by CDV, andthereafter patterned. Thus, an oxide film is formed on the gateelectrode 35 for serving as a stopper film for impurity introductioninto the gate electrode 35. This oxide film may be employed as a maskfor ion-implanting boron and nitrogen only into the source/drain regions6. While boron ions are implanted before patterning of the gateelectrode 35 in the aforementioned fabrication process for the PMOStransistor according to the third embodiment, this implantation step mayalternatively be omitted so that boron ions are implanted into the gateelectrode 35 simultaneously with implantation of boron ions into thesource/drain regions 6.

While the third embodiment has been described with reference to a PMOStransistor alone, the present invention is not restricted to this butalso applicable to a CMOS transistor including a PMOS transistor.Further, titanium silicide layers 8 may be formed on the source/drainregions 6 as shown in FIG. 33, in order to reduce the gate electrode 35and the source/drain regions 6 in resistance. Such titanium silicidelayers 8 can be readily formed on the gate electrode 35 and thesource/drain regions 6 through a titanium salicide step, after the stepshown in FIG. 26 or 32.

Effects of the PMOS transistor according to the third embodiment aresimilar to those of the first and second embodiments. According to thethird embodiment, further, it is possible to change nitrogen profiles ofthe gate electrode 35 and the source/drain regions 6 by separatelycarrying out nitrogen implantation steps for the gate electrode 35 andthe source/drain regions 6 although the number of steps is increased,thereby optimizing the respective nitrogen profiles. Thus, it ispossible to further effectively prevent boron from punching through thegate oxide film 36 and from diffusion in the source/drain regions 6.

With reference to FIG. 34, an NMOS transistor according to a fourthembodiment of the present invention is now described. As shown in FIG.34, element isolation oxide film 7 are formed on a main surface of aP-type silicon substrate 40 in the NMOS transistor according to thefourth embodiment. Further, N⁻-type source/drain regions 43 are formedon an active region which is held between the element isolation oxidefilm 7 at a prescribed space, to hold a channel region 10 therebetween.N⁺-type source/drain regions 44 are formed to be adjacent to the N⁻-typesource/drain regions 43. The N⁻-type source/drain regions 43 and theN⁺-type source/drain regions 44 form an NMOS transistor of an LDD(lightly-doped drain) structure. Referring to FIG. 35, it is understoodthat nitrogen is deposited in a gate oxide film 42 in the NMOStransistor according to the fourth embodiment.

With reference to FIGS. 36 to 41, a fabrication process for the NMOStransistor according to the fourth embodiment is now described.

First, element isolation oxide film 7 are formed on a P-type siliconsubstrate 40 through an ordinary element isolation step, as shown inFIG. 36. An oxide film 42 a of about 100 Å in thickness is formed bythermal oxidation, and thereafter a polysilicon film 41 a of about 2000Å in thickness is formed by CDV.

Then, nitrogen ions are implanted under conditions of 20 keV and1×10¹⁶/cm², so that a range center comes to an upper portion of thepolysilicon film 41 a. Thereafter arsenic ions are implanted into thepolysilicon film 41 a under conditions of 30 keV and 4×10¹⁵/cm², asshown in FIG. 38. Then, the polysilicon film 41 a is patterned byphotolithography and anisotropic etching, to form a gate electrode 41.

Then, arsenic ions are implanted into source/drain forming regions at anangle of incidence of 45°, as shown in FIG. 39. In more concrete terms,the P-type silicon substrate 40 is rotated, so that arsenic ions areimplanted into the same under conditions of 50 keV and 4×10¹³/cm².Thereafter an oxide film of about 800 Å in thickness is formed by CDV,and this oxide film is etched back. Thus, side wall oxide films 5 and agate oxide film 42 b are formed as shown in FIG. 40. Thereafter arsenicions are implanted into N⁺-type source/drain forming regions underconditions of 50 keV and 4×10¹⁵/cm².

Finally, heat treatment is carried out at 850° C. for about 20 minutes,thereby activating the impurities as implanted. Thus, the N⁻-typesource/drain regions 43, the N⁺-type source/drain regions 44, the gateelectrode 41 and nitrogen-doped regions 30 are formed as shown in FIG.34. Nitrogen segregates in the gate oxide film 42 b, whereby a gateoxide film 36 having a nitrogen concentration peak as shown in FIG. 35is formed. A projection range R_(P) of nitrogen is set to come to aposition of the gate electrode 41 upward beyond a position separated by5×ΔR_(P) from the interface between the N⁺-type gate electrode 41 andthe gate oxide film 42 toward the N⁺-type source/drain regions 44,assuming that ΔR_(P) represents its standard deviation.

While arsenic is ion-implanted into the polysilicon film 41 a to formthe gate electrode 41 which is doped in an N type in the abovedescription, the present invention is not restricted to this but thegate electrode 41 which is doped in an N type may alternatively beformed through a doped polysilicon film which is doped with phosphorusby about 5×10²⁰/cm³. While the fourth embodiment has been described withreference to an NMOS transistor alone, this embodiment is alsoapplicable to a CMOS transistor including an NMOS transistor.

As to an effect of the fourth embodiment, nitrogen is deposited in thegate oxide film 42 in later heat treatment, since the gate electrode 41is doped with nitrogen in its upper portion. Consequently, aninterfacial level is reduced at the interface between the gate oxidefilm 42 which is formed by a silicon oxide film and the P-type substrate40 which is made of silicon. Thus, the gate oxide film 42 is improved inreliability, while hot carrier resistance is effectively improved.Evaluation of reliability of the gate oxide film 42 is as describedabove with reference to FIG. 9. FIG. 42 shows dependence of a thresholdvoltage of the NMOS transistor, caused by hot carrier injection, on thenitrogen injection rate. Referring to FIG. 42, change of the thresholdvoltage was measured after a constant stress voltage was applied for 100seconds. Change of the threshold voltage is reduced when the injectionrate of nitrogen into the gate electrode 41 is increased. Thus, it isunderstood that hot carrier resistance of the NMOS transistor isimproved when the gate electrode 41 is doped with nitrogen so thatnitrogen is deposited in the gate oxide film 42.

A nitrogen concentration peak in the nitrogen-doped region 30 in each ofthe gate electrode 41 and the gate oxide film 42 is preferably set in arange of 10¹⁹/cm³ to 10²¹/cm³. Therefore, the injection rate of nitrogenions in the fabrication steps may be set in a range of 10¹⁴/cm² to10¹⁶/cm².

With reference to FIG. 43, an NMOS transistor according to a fifthembodiment of the present invention is now described. In the NMOStransistor according to the fifth embodiment, nitrogen-doped regions 30are formed in N⁺-type source/drain regions 44. Referring to FIG. 44, itis understood that junction planes of N⁻-type source/drain regions 43are not doped with nitrogen but the nitrogen-doped regions 30 exist inthe N⁺-type source/drain regions 44 which are formed by doping arsenic.

With reference to FIGS. 45 to 48, a fabrication process for the NMOStransistor according to the fifth embodiment shown in FIG. 43 is nowdescribed.

First, element isolation oxide film 7 and an oxide film 2 a of about 100Å in thickness are formed on a P-type silicon substrate 40, as shown inFIG. 45. Further, a polysilicon film (not shown) which is doped withphosphorus by about 5×10²⁰/cm³ is formed on the oxide film 2 a in athickness of about 2000 Å by CDV. An oxide film (not shown) of about2000 Å in thickness is formed on this polysilicon film. The oxide filmand the polysilicon film are patterned in the form of a gate electrodeby photolithography and anisotropic etching. Thus, an oxide film 4 and agate electrode 3 are formed as shown in FIG. 45.

Then, the gate electrode 3, the oxide film 4 and the element isolationoxide film 7 are employed as masks to ion-implant arsenic ions into theP-type silicon substrate 40 at an angle of incidence of 45°, as shown inFIG. 46. In more concrete terms, the P-type silicon substrate 40 isrotated so that the arsenic ions are implanted into the same at theangle of incidence of 45° under conditions of 50 keV and 4×10¹³/cm².Thereafter an oxide film (not shown) of about 800 Å in thickness isformed by CDV, and this oxide film is etched back. Thus, side wall oxidefilms 5 and a gate oxide film 2 are formed as shown in FIG. 47. Further,the side wall oxide films 5 and the element isolation oxide film 7 areemployed as masks to implant nitrogen ions into the P-type siliconsubstrate 40 under conditions of 10 keV and 2×10¹⁵/cm². Further, theside wall oxide films 5 and the element isolation oxide film 7 are againemployed as masks to implant arsenic ions into the P-type siliconsubstrate 40 under conditions of 50 keV and 4×10¹⁵/cm². Heat treatmentis carried out at 850° C. for about 20 minutes, thereby activating theimpurities as implanted. Thus, the N⁻-type source/drain regions 43, theN⁺-type source/drain regions 44 and the nitrogen-doped regions 30 areformed as shown in FIG. 42.

Conditions for nitrogen implantation in the aforementioned fabricationprocess for the NMOS transistor according to the fifth embodiment areidentical to those described with reference to the first embodiment.Namely, nitrogen is implanted at such energy that its projection rangeis smaller than that of arsenic.

While the gate electrode 3 is formed by a polysilicon film which isdoped with phosphorus in the aforementioned fabrication process, thepresent invention is not restricted to this but a non-doped polysiliconfilm may be formed so that an N-type impurity is implanted into the samefor forming a gate electrode. Further, a gate electrode having a layeredstructure of a metal silicide film and a polysilicon film may beemployed so that the gate electrode is reduced in sheet resistance. Asshown in FIG. 49, further, titanium silicide films 8 may be formed onthe N⁺-type source/drain regions 44, so that the source/drain regions 44are reduced in resistance. As to an effect of the fifth embodiment, theN⁺-type source/drain regions 44 are doped with nitrogen and hencediffusion of arsenic is suppressed. Namely, the relation between boronand nitrogen described with reference to the first embodimentalso-applies to relation between arsenic and nitrogen. Thus, it ispossible to suppress diffusion of arsenic by mutually diffusing arsenicand nitrogen, thereby forming the source/drain regions with junctionplanes which are shallower than those of the prior art.

A nitrogen concentration peak in the nitrogen-doped region 30 of eachN⁺-type source/drain region 44 is preferably set in a range of 10¹⁹/cm³to 10²¹/cm³. Therefore, the injection rate for nitrogen ions in thefabrication process may be set in a range of 10¹⁴/cm² to 10¹⁶/cm². Theaforementioned effect cannot be attained if the nitrogen concentrationpeak is lower than 10¹⁹/cm³, while an activation rate of arsenic isreduced and hence the N⁺-type source/drain region 44 is increased inresistance if the nitrogen concentration peak is higher than 10²¹/cm³.

With reference to FIG. 50, a dual gate CMOS transistor according to asixth embodiment of the present invention is now described. According tothe sixth embodiment, element isolation oxide film 12 are formed on amain surface of a P-type silicon substrate 11. Further, an N well 13 anda P well 14 are formed on the main surface of the P-type siliconsubstrate 11, to be adjacent to each other. Source/drain regions 21 areformed on a main surface of the N well 13 at a prescribed space to holda channel region 10 therebetween. A P⁺-type gate electrode 50 is formedon the channel region 10 through a gate oxide film 47. Titanium silicidefilms 23 are formed on upper surfaces of the P⁺-type gate electrode 50and the source/drain regions 21. Side wall oxide film s 20 are formed onside surfaces of the P⁺-type gate electrode 50. Further, nitrogen-dopedregions 30 (slant portions) are formed in the source/drain regions 21,the gate oxide film 47 and the P⁺-type gate electrode 50.

On the other hand, N⁻-type source/drain regions 52 are formed on a mainsurface of the P well 14 at a prescribed space for holding a channelregion 10 therebetween. N⁺-type source/drain regions 53 are formed incontinuation to the N⁻-type source/drain regions 52. An N⁺-type gateelectrode 51 is formed on the channel region 10 through a gate oxidefilm 48. Titanium silicide films 23 are formed on upper surfaces of theN⁺-type source/drain regions 53 and the N⁺-type gate electrode 51. Sidewall oxide films 20 are formed on side surfaces of the N⁺-type gateelectrode 51 and the titanium silicide film 23 provided thereon.According to the sixth embodiment, the titanium silicide films 23 areadapted to reduce the source/drain regions in resistance, whileproviding the gate electrodes 50 and 51 of polycide structures.

With reference to FIGS. 51 to 59, an exemplary fabrication process forthe dual gate CMOS transistor according to the sixth embodiment is nowdescribed. First, an N well 13 and a P well 14 are formed in a P-typesilicon substrate 11, as shown in FIG. 51. Thereafter element isolationoxide film 12 are formed on the P-type silicon substrate 11 by anordinary element isolation step. Then, oxide films 49 of about 100 Å inthickness are formed on the N well 13 and the P well 14 by thermaloxidation. Then, a polysilicon film 55 of about 2000 Å in thickness isformed on the oxide films 49 and the element isolation oxide film 12 byCDV.

Then, nitrogen ions are implanted into the polysilicon film 55 underconditions of 20 keV and 4×10¹⁵/cm², so that a range center comes to anupper portion of the polysilicon film 55, as shown in FIG. 52.Thereafter a PMOS transistor forming region is covered with a resistfilm 60 as shown in FIG. 53, and the resist film 60 is employed as amask to implant arsenic ions into a portion of the polysilicon film 55provided in an NMOS transistor forming region under conditions of 30 keVand 4×10¹⁵/cm². Thereafter the resist film 60 is removed.

Then, the NMOS transistor forming region is covered with a resist film61 as shown in FIG. 54, and the resist film 61 is employed as a mask toimplant boron ions into a portion of the polysilicon film 55 provided inthe PMOS transistor forming region under conditions of 20 kev and4×10¹⁵/cm². Thereafter the resist film 61 is removed. The polysiliconfilm 55 is patterned by photolithography and anisotropic etching,thereby forming polysilicon films 50 a and 51 a as shown in FIG. 55.

Then, the PMOS transistor forming region is covered with a resist film62 as shown in FIG. 56, and the resist film 62 is employed as a mask toimplant arsenic ions into N⁻-type source/drain regions on the P well 14,as shown in FIG. 56. In more concrete terms, the P-type siliconsubstrate 11 is rotated so that the arsenic ions are implanted into thesame at an angle of incidence of 45° under conditions of 50 keV and4×10¹³/cm². Thereafter the resist film 62 is removed.

Then, an oxide film of about 800 Å in thickness is formed on the overallsurface by CDV, and this oxide film is etched back to form side walloxide films 20 and gate oxide films 47 a and 48 a, as shown in FIG. 57.Then, the PMOS transistor forming region is covered with a resist film63, as shown in FIG. 58. The resist film 63 is employed as a mask toimplant arsenic ions into N⁺-type source/drain forming regions on the Pwell 14 under conditions of 50 keV and 4×10¹⁵/cm². Thereafter the resistfilm 63 is removed.

Then, the NMOS transistor forming region is covered. with a resist film64, as shown in FIG. 59. The resist film 64 is employed as a mask toimplant nitrogen ions under conditions of 10 keV and 2×10¹⁵/cm², andthen boron ions are implanted under conditions of 10 keV and 4×10¹⁵/cm².Thereafter the resist film 64 is removed. Heat treatment is carried outat 850° C. for about 20 minutes, thereby activating the impurities asimplanted. Thus, the source/drain regions 21, the P⁺-type polysiliconfilm 50, the N⁻-type source/drain regions 52, the N⁺-type source/drainregions 53, the N⁺-type gate electrode 51 and the nitrogen-doped regions30 are formed as shown in FIG. 50. During the heat treatment, nitrogenwhich is doped in the upper portions of the polysilicon films 50 a and51 a is thermally diffused. At this time, nitrogen segregates in thegate oxide films 47 a and 48 a, thereby forming gate oxide films 47 and48 having nitrogen concentration peaks therein. Then, titanium layers(not shown) of about 500 Å in thickness are formed by sputtering, andsubjected to heat treatment at 700° C. for 30 seconds. Thus, thetitanium silicide films 23 (see FIG. 50) are formed on the P⁺-typepolysilicon films 50 and 51, the P⁺-type source/drain regions 21, andthe N⁺-type source/drain regions 53. Thereafter unreacted titaniumlayers are removed from the oxide films, thereby forming the dual gateCMOS transistor according to the sixth embodiment.

With reference to FIGS. 60 to 64, another exemplary fabrication processfor the dual gate CMOS transistor according to the sixth embodimentshown in FIG. 50 is now described. First, elements up to a polysiliconfilm 55 are formed through a process which is similar to that shown inFIG. 51. Thereafter a PMOS transistor forming region is covered with aresist film 60, as shown in FIG. 60. This resist film 60 is employed asa mask to implant nitrogen ions into the polysilicon film 55 underconditions of 25 keV and 1×10¹⁶/cm², so that a range center comes to anupper portion of the polysilicon film 55.

Then, the resist film 60 is employed as a mask to further implantarsenic ions into the polysilicon film 55 under conditions of 30 keV and4×10¹⁵/cm², as shown in FIG. 61. Thereafter the resist film 60 isremoved.

Then, an NMOS transistor forming region is covered with a resist film61, as shown in FIG. 62. This resist film 61 is employed as a mask toimplant nitrogen ions into the polysilicon film 55 under conditions of15 keV and 4×10¹⁵/cm², so that a range center comes to an upper portionof the polysilicon film 55. Then, the resist film 61 is employed as amask to further implant boron ions into the polysilicon film 55 underconditions of 20 keV and 4×10¹⁵/cm². Thereafter the resist film 61 isremoved. The polysilicon film 55 is patterned by photolithography andanisotropic etching, thereby forming polysilicon films 50 a and 51 ashown in FIG. 64. Subsequent steps are similar to those shown in FIGS.56 to 59.

While the step of doping the polysilicon film 50 a with boron is carriedout independently of that of doping the source/drain regions with boronin each of the aforementioned two fabrication processes, the polysiliconfilm 50 a may alternatively be doped with boron in the step of dopingthe source/drain regions with boron. Further, the step of doping thepolysilicon film 50 a with arsenic may also be carried out with the stepof doping the N⁻-type or N⁺-type source/drain regions with arsenic.

Effects of the sixth embodiment are now described. In the PMOStransistor region, the P⁺-type polysilicon film 50 and the P-typesource/drain regions 21 are doped with nitrogen, whereby effects similarto those described above with reference to the first and secondembodiments can be attained. In the NMOS transistor region, further, theN⁺-type polysilicon film 51 is doped with nitrogen, whereby an effectsimilar to those described with reference to the fourth embodiment canbe attained. When nitrogen ions are implanted into the polysilicon film50 a and the N⁺-type polysilicon film 51 in separate steps, it ispossible to optimize nitrogen profiles thereof in response to theproperties of the ions which are implanted into these polysilicon films50 a and 51 a. Thus, it is possible to further suppress punch-through ofboron from the P⁺-type polysilicon film 50 in the PMOS transistor regionand generation of an interfacial level in the interface between the gateoxide film 47 and the silicon substrate 11 in the NMOS transistorregion.

With reference to FIG. 65, a dual gate CMOS transistor according to aseventh embodiment of the present invention is now described. Accordingto the seventh embodiment, a gate electrode of a PMOS transistor has atwo-layer structure of a P⁺-type polysilicon film 50 and a tungstensilicide film 70 which is formed thereon, while a gate electrode of anNMOS transistor also has a two-layer structure of an N⁺-type polysiliconfilm 51 and a tungsten silicide film 71 which is formed thereon.Further, oxide films 19 are formed on the tungsten silicide films 70 and71. Side wall oxide films 20 are formed on side surfaces of the gateelectrodes.

According to the seventh embodiment, nitrogen-doped regions 30 areformed in the gate electrode and a gate oxide film 47 of the NMOStransistor, and P⁺-type source/drain regions 21 of the PMOS transistor.Referring to FIG. 66, it is understood that a nitrogen concentrationpeak exists in the interface between the P⁺-type polysilicon film 50 andthe tungsten silicide film 70 and nitrogen is deposited in the gateoxide film 47, in the gate electrode of the PMOS transistor. Referringto FIG. 67, on the other hand, it is understood that a nitrogenconcentration peak exists in the interface between the N⁺-typepolysilicon film 51 and the tungsten silicide film 71 and nitrogen isdeposited in the gate oxide film 48, in the gate electrode of the NMOStransistor.

With reference to FIGS. 68 to 76, a fabrication process for the dualgate CMOS transistor according to the seventh embodiment shown in FIG.65 is now described. First, an N well 13 and a P well 14 are formed on aP-type silicon substrate 11 as shown in FIG. 68, and element isolationoxide film 12 are formed on the P-type silicon substrate 11 through anordinary element isolation step. Thereafter an oxide film 49 of about100 Å in thickness is formed by thermal oxidation. A polysilicon film 55of about 2000 Å in thickness is formed by CDV.

Then, a tungsten silicide film 72 of about 1000 Å in thickness is formedby sputtering, as shown in FIG. 69.

Then, nitrogenions are implanted under conditions of 40 keV and1×10¹⁶/cm², so that a range center comes to a position which is close tothe interface between the polysilicon film 55 and the tungsten silicidefilm 72, as shown in FIG. 70. Then, a PMOS transistor forming region iscovered with a resist film 60, as shown in FIG. 71. The resist film 60is employed as a mask to implant arsenic ions into a portion of thepolysilicon film 55 in an NMOS transistor forming region underconditions of 120 keV and 4×10¹⁵/cm². Thereafter the resist film 60 isremoved.

Then, the NMOS transistor forming region is covered with a resist film61, as shown in FIG. 72. The resist film 61 is employed as a mask toimplant boron ions into a portion of the polysilicon film 55 in the PMOStransistor forming region under conditions of 30 keV and 4×10¹⁵/cm².Thereafter the resist film 61 is removed. An oxide film (not shown) ofabout 2000 Å in thickness is formed by CDV, and the oxide film, thetungsten silicide film 72 and the polysilicon film 55 are patterned intothe form of gate electrodes. Thus, oxide films 19, tungsten silicidefilms 70 a and 71 a and polysilicon films 50 a and 51 a are formed asshown in FIG. 73.

Then, the PMOS transistor forming region is covered with a resist film62, as shown in FIG. 74. The resist film 62 is employed as a mask toimplant arsenic ions into N⁻-type source/drain regions. This ionimplantation is carried out at an angle of incidence of 45° underconditions of 50 keV and 4×10¹³/cm², while rotating the P-type siliconsubstrate 11. Thereafter the resist film 62 is removed.

Then, an oxide film of about 800 Å in thickness is formed by CDV, andthis oxide film is etched back to form side wall oxide films 20 and gateoxide films 47 a and 48 a as shown in FIG. 75. The PMOS transistorforming region is covered with a resist film 63, and this resist film 63is employed as a mask to implant arsenic ions into N⁺-type source/drainregions under conditions of 50 keV and 4×10¹³/cm². Thereafter the resistfilm 63 is removed.

Then, the NMOS transistor forming region is covered with a resist film64, as shown in FIG. 76. The resist film 64 is employed as a mask toimplant nitrogen into source/drain regions of the PMOS transistor underconditions of 10 keV and 2×10¹⁵/cm², and thereafter boron ions areimplanted under conditions of 10 keV and 4×10¹³/cm². Thereafter theresist film 64 is removed. Heat treatment is carried out at 850° C. forabout 20 minutes, thereby activating the impurities as implanted. Thus,the tungsten silicide films 70 and 71, the P⁺-type polysilicon film 50,the N⁺-type polysilicon film 51, the source/drain regions 21, the N-typesource/drain regions 52, the N⁺-type source/drain regions 53 and thenitrogen-doped regions 30 are formed as shown in FIG. 65. During theheat treatment, nitrogen which is doped into the interface between thepolysilicon film 50 a and the tungsten silicide film 70 a and thatbetween the polysilicon film 51 a and the tungsten silicide film 71 a isthermally diffused. At this time, nitrogen segregates in the gate oxidefilms 47 a and 48 a, thereby forming gate oxide films 47 and 48 havingnitrogen concentration peaks as shown in FIGS. 66 and 67.

As to an effect of the seventh embodiment, portions close to theinterfaces between the P⁺-type polysilicon film 50 and the tungstensilicide film 70 and between the N⁺-type polysilicon film 51 and thetungsten silicide film 71 are doped with nitrogen, whereby boron andarsenic are inhibited from diffusion in the tungsten silicide films 70and 71 respectively. Namely, nitrogen having a larger diffusioncoefficient as compared with boron and arsenic occupies diffusion pathsin advance. Consequently, it is possible to inhibit boron and arsenicfrom diffusion into the tungsten silicide films 70 and 71 respectively,thereby effectively suppressing fluctuation of a threshold voltagecaused by change in working function resulting from mutual diffusion ofboron and arsenic. While the nitrogen-doped regions 30 are formed in thesource/drain regions 21 according to this embodiment, thesenitrogen-doped regions 30 may not be formed when the source/drainregions 21 are formed by implanting boron fluoride ions under conditionsof 20 keV an 4×10¹⁵/cm², for example.

With reference to FIG. 77, a PMOS-TFT according to an eighth embodimentof the present invention is now described. In the PMOS-TFT according tothe eighth embodiment, an insulating film 102 is formed on asemiconductor substrate 101. A gate electrode 103 is formed on aprescribed region of the insulating film 102. A gate insulating film 104is formed on the gate electrode 103 and the insulating film 102. Apolycrystalline silicon layer 105 is formed on the gate insulating film104. The polysilicon layer 105 is provided with a source region 105 band a drain region 105 c at a prescribed space, to hold a channel region105 a therebetween. Referring to FIG. 77, slant portions shownitrogen-doped regions 110. FIG. 78 illustrates an impurity profile of adepth direction in a section taken along the line a—a in FIG. 77. FIG.79 illustrates an impurity profile of a depth direction in a sectiontaken along the line b—b in FIG. 77. Referring to FIGS. 78 and 79, it isunderstood that the nitrogen-doped regions 110 reach portions of thechannel region 105 a located outside end surfaces of the source anddrain regions 105 b and 105 c.

With reference to FIGS. 80 to 82, a fabrication process for the PMOS-TFTaccording to the eighth embodiment shown in FIG. 77 is now described. Aninsulating film 102 is formed on a surface of a semiconductor substrate101, and then a non-doped polysilicon layer (not shown) is formed on theinsulating film 102 in a thickness of about 2000 Å by CDV. Boron ision-implanted into the non-doped polysilicon layer for forming apolysilicon layer which is doped in a P type, and the doped polysiliconlayer is patterned in the form of a gate electrode by photolithographyand anisotropic etching. Thus, a gate electrode 103 shown in FIG. 80 isformed. Then, a gate insulating film 104 of about 100 Å in thickness isformed by thermal oxidation, and a non-doped polysilicon layer (notshown) is formed on the gate insulating film 104 in a thickness of about2000 Å by CDV. In order to control a threshold voltage, arsenic ision-implanted into the non-doped polysilicon layer under conditions of50 keV and 1×10¹² to 1×10¹³/cm², thereby forming a polysilicon layerwhich is doped in an N type. Then, the polysilicon layer is patterned ina desired shape by photolithography and anisotropic etching. Thus, apolycrystalline silicon layer 105 is formed as shown in FIG. 80.

Then, a resist film 107 is formed on the channel region 105 a byphotolithography, as shown in FIG. 81. The resist film 107 is employedas a mask, to ion-implant nitrogen into the polysilicon layer 105. Theion implantation of nitrogen is carried out at an angle of incidence of15 to 60° under conditions of 10 keV and 2×10¹⁵/cm², while rotating thesemiconductor substrate 101. Thereafter boron fluoride ions areimplanted into the polysilicon layer 105 under conditions of 10 keV and4×10¹⁵/cm² as shown in FIG. 82. Heat treatment is carried out at 850° C.for about 20 minutes, thereby activating the impurities as implanted.Thus, nitrogen-doped regions 110 are formed simultaneously withformation of the P-type source and drain regions 105 b and 105 c shownin FIG. 77.

Relation between nitrogen implantation conditions and implantationconditions for the source/drain regions is now described. Implantationenergy for nitrogen is so set that a range R_(P) of nitrogen is smallerthan that of boron fluoride. If the nitrogen-doped regions 110 as formedare deeper than junction planes of the source/drain regions, crystaldefects formed in nitrogen implantation are contained in depletionlayers which are formed in the junction planes of the source/drainregions, leading to generation of a junction leakage current. Therefore,the implantation energy for nitrogen must be set in the aforementionedmanner.

While boron fluoride is ion-implanted into the gate electrode 103 in theabove description, boron may alternatively be ion-implanted. Further, anN-type gate electrode may be employed in place of a P-type gateelectrode. While boron fluoride ions are implanted also into the P-typesource/drain regions, boron ions may alternatively be implanted into thesame. While the above embodiment has been described with reference to aP-channel MOS-TFT, the present invention is also applicable to aCMOS-TFT including a P-channel MOS-TFT as a part thereof.

As to an effect of the eighth embodiment, the source and drain regions105 b and 105 c are doped with nitrogen, whereby diffusion of boron issuppressed. Namely, nitrogen is in the same hole diffusion mechanism asboron, and has a larger diffusion coefficient than boron. Thus, nitrogenoccupies holes, which are diffusion paths, due to mutual diffusion ofnitrogen and boron, whereby it is possible to suppress diffusion ofboron. Therefore, it is possible to suppress transverse diffusion ofboron into the channel region 105 a, thereby increasing an effectivegate length. Thus, it is possible to prevent a punch-through phenomenoncaused by a short channel effect. Further, it is possible to furthersuppress transverse diffusion of boron by carrying out oblique rotationimplantation of nitrogen.

A ninth embodiment of the present invention is now described. While thepresent invention is applied to a PMOS-TFT in the eighth embodiment, thepresent invention is applied to an N-channel MOS-TFT (hereinafterreferred to as “NMOS-TFT”) in the ninth embodiment. In order to formsuch an NMOS-TFT, the conductivity types of the implanted impurities maybe reversed to those employed for forming the PMOS-TFT in FIG. 77.Namely, the gate electrode 103, the source region 105 b and the drainregion 105 c are doped in N types, while the channel region 105 a isdoped in a P type. FIG. 83 shows an impurity profile of a depthdirection in a section taken along the line a—a, in a case of formingthe TFT shown in FIG. 77 as an N-channel type one. FIG. 84 shows animpurity profile of a depth direction in a section taken along the lineb—b. Referring to FIGS. 83 and 84, it is understood that thenitrogen-doped regions 110 reach portions of the channel region 105 alocated outside end surfaces of the source and drain regions 105 b and105 c.

A fabrication process for the NMOS-TFT according to the ninth embodimentis basically identical to that for the PMOS-TFT according to the eighthembodiment. Therefore, this process is described with reference to FIGS.80 to 82. However, this embodiment is different in impurity implantationcondition from the eighth embodiment, and hence the impurity shown inparentheses in FIG. 82 is employed in the ninth embodiment. First, aninsulating film 102 is formed on a semiconductor substrate 101, and anon-doped polysilicon layer is formed on the insulating film 102 in athickness of about 2000 Å by CDV. Thereafter arsenic is ion-implantedinto the non-doped polysilicon layer, thereby forming a polysiliconlayer which is doped in an N type. The doped polysilicon layer ispatterned in the form of a gate electrode by photolithography andanisotropic etching. Thus, a gate electrode 103 is formed. Then, a gateinsulating film 104 of about 100 Å in thickness is formed by thermaloxidation, and a non-doped polysilicon layer is formed on the gateinsulating film 104 in a thickness of about 2000 Å by CDV. In order tocontrol a threshold voltage, boron fluoride is ion-implanted into thenon-doped polysilicon layer under conditions of 20 keV and 1×10¹² to1×10¹³/cm². Thus, a polysilicon layer which is doped in a P type isformed. Then the polysilicon layer is patterned by photolithography andanisotropic etching, to leave regions for defining a channel region, asource region and a drain region. Thus, a polysilicon layer 105 having aprescribed shape is formed as shown in FIG. 80.

Then, a resist film 107 is formed on the channel region 105 a byphotolithography, as shown in FIG. 81. The resist film 107 is employedas a mask to ion-implant nitrogen into the polysilicon layer 105. Thision implantation is carried out at an angle of incidence of 15 to 60°under conditions of 10 keV and 2×10¹⁵/cm², while rotating thesemiconductor substrate 101. Then, arsenic is ion-implanted into thepolysilicon layer 105 under conditions of 30 keV and 4×10¹⁵/cm², asshown in FIG. 82. Heat treatment is carried out at 850° C. for about 20minutes, thereby activating the impurities as implanted. Thus, N-typesource and drain regions 105 b and 105 c as well as nitrogen-dopedregions 110 are formed. Also in the ninth embodiment, relation betweennitrogen implantation conditions and implantation conditions for thesource/drain regions is similar to that in the eighth embodiment.Namely, implantation energy for nitrogen is so set that a range R_(P) ofnitrogen is smaller than that of arsenic.

While arsenic implantation is employed for the gate electrode in theabove description, phosphorus may be employed in place of arsenic withno problem. Further, a P-type gate electrode may be employed in place ofthe N-type gate electrode. While arsenic is implanted also in the N-typesource/drain regions, phosphorus may be employed in place of arsenic.While the ninth embodiment has been described with reference to anN-channel MOS-TFT, this embodiment is also applicable to a CMOS-TFTincluding an NMOS-TFT.

As to an effect of the ninth embodiment, the N-type source/drain regionsare doped with nitrogen similarly to the eighth embodiment, whereby itis possible to suppress diffusion of arsenic or phosphorus. Namely, thedescription made on the relation between boron and nitrogen withreference to the eighth embodiment is also applicable to that betweenarsenic and boron, or that between phosphorus and nitrogen. Therefore,it is possible to suppress diffusion of arsenic by mutually diffusingnitrogen and arsenic. Thus, it is possible to suppress transversediffusion of arsenic or phosphorus in the channel region by an action ofnitrogen, thereby increasing an effective gate length. Consequently, itis possible to prevent a punch-through phenomenon caused by a shortchannel effect. Further, it is possible to further suppress transversediffusion of phosphorus or arsenic by implanting nitrogen in an obliquerotation manner.

With reference to FIG. 85, a PMOS-TFT according to a tenth embodiment ofthe present invention is now described. According to the tenthembodiment, nitrogen-doped regions 110 are formed not only in source anddrain regions 105 b and 105 c but a polysilicon layer 105 and a gateinsulating film 111. FIG. 86 shows an impurity profile of a depthdirection in a section taken along the line a—a in FIG. 85. An impurityprofile of a depth direction in a section taken along the line b—b inFIG. 85 is identical to that shown in FIG. 78. Referring to FIG. 86, itis understood that nitrogen is deposited in the gate insulating film111.

With reference to FIGS. 87 to 89, a fabrication process for the PMOS-TFTaccording to the tenth embodiment is now described. First, elements upto a gate electrode 103 are formed as shown in FIG. 87, through aprocess similar to the fabrication process described with reference tothe eighth embodiment. Then, a gate insulating film 111 a of about 100 Åin thickness is formed by thermal oxidation, and a non-doped polysiliconlayer 106 is formed in a thickness of about 2000 Å by CDV, to cover thegate insulating film llla. Then, nitrogen is ion-implanted into thenon-doped polysilicon layer 106 at an angle of incidence of 15 to 60°under conditions of 10 keV and 2×10¹⁵/cm², while rotating thesemiconductor substrate 101. In order to control a threshold voltage,arsenic is ion-implanted into the polysilicon layer 106 under conditionsof 50 keV and 1×10¹² to 1×10¹³/cm² (this step is now shown). Thepolysilicon layer 106 is patterned by photolithography and anisotropicetching, thereby forming a polysilicon layer 105 having a desired shapeas shown in FIG. 88.

Then, a resist film 107 is formed on a channel region byphotolithography, as shown in FIG. 89. This resist film 107 is employedas a mask to ion-implant boron fluoride under conditions of 30 keV and4×10¹⁵/cm². Thus, P-type source and drain regions 105 b and 105 c andnitrogen-doped regions 110 are formed as shown in FIG. 85. In this heattreatment, nitrogen implanted into the polysilicon layer 105 isthermally diffused while nitrogen segregates in the gate insulating filmlila. Thus, a gate insulating film 111 having the nitrogen-doped region110 is formed.

Relation between the nitrogen implantation conditions and implantationconditions for the source/drain region is identical to that in the firstembodiment. Namely, implantation energy for nitrogen is so set that arange R_(P) of nitrogen is smaller than that of boron fluoride. Whileoblique rotation implantation of nitrogen is employed in theaforementioned fabrication process, nitrogen may alternatively bevertically implanted to be thereafter diffused in a channel portiondefined on a side wall portion of the gate electrode 103 by later heattreatment.

As to an effect of the tenth embodiment, an interfacial level in theinterface between the polysilicon layer 105 and the gate insulating film111 is reduced since nitrogen segregates in the gate insulating film111. Thus, it is possible to improve reliability of the gate insulatingfilm 111. Namely, it is possible to inhibit hot carriers generated on adrain end from being trapped in the gate insulating film 111 by suchreduction of the interfacial level, thereby improving hot carrierresistance. Further, the source/drain regions are also doped withnitrogen, whereby it is possible to prevent occurrence of apunch-through phenomenon caused by diffusion of the impurity forming thesource/drain regions.

An NMOS-TFT according to an eleventh embodiment of the present inventionis now described. While the present invention is applied to a PMOS-TFTin the tenth embodiment, the same is applied to an NMOS-TFT according tothe eleventh embodiment. In order to form such an NMOS-TFT, theconductivity types of the implanted impurities may be reversed to thosefor forming the PMOS-TFT in FIG. 85. Namely, the gate electrode 103 andthe source and drain regions 105 b and 105 c are doped with an N-typeimpurity, while the channel region 105 a is doped with a P-typeimpurity. FIG. 90 shows an impurity profile of a depth direction in asection taken along the line a—a in FIG. 85, in a case of forming theTFT shown in FIG. 85 as an N-channel type one. An impurity profile of adepth direction in a section taken along the line b—b is identical tothat shown in FIG. 83. Referring to FIG. 90, it is understood thatnitrogen is deposited in the gate insulating film 111 in the eleventhembodiment.

A fabrication process for the NMOS-TFT according to the eleventhembodiment is now described. The fabrication process according to theeleventh embodiment is basically identical to that for the PMOS-TFTaccording to the tenth embodiment. Therefore, this process is describedwith reference to FIGS. 87 to 89. However, this embodiment is differentin impurity implantation condition from the tenth embodiment, and hencethe impurity shown in parentheses in FIG. 89 is employed in the eleventhembodiment.

First, elements up to a gate electrode 103 are formed through stepssimilar to those described with reference to the eighth embodiment.Then, a gate insulating film 111 a of about 100 Å in thickness is formedby thermal oxidation, and thereafter a non-doped polysilicon layer isformed on the gate insulating film 111 a by CVD in a thickness of about2000 Å. Thereafter nitrogen is ion-implanted into the non-dopedpolysilicon layer at an angle of incidence of 15 to 60° under conditionsof 10 keV and 2×10¹⁵/cm² while rotating the semiconductor substrate 101.In order to control a threshold voltage, boron fluoride is ion-implantedinto the polysilicon layer under conditions of 30 keV and 1×10¹² to1×10¹³/cm². Then, the polysilicon layer is patterned in a prescribedshape by photolithography and anisotropic etching, thereby forming apolysilicon layer 105 of a prescribed shape as shown in FIG. 88. Aresist film 107 is formed on a channel region by photolithography, asshown in FIG. 89. The resist film 107 is employed as a mask toion-implant arsenic into the polysilicon layer 105 under conditions of30 keV and 4×10¹⁵/cm². Heat treatment is carried out at 850° C. forabout 20 minutes, thereby activating the impurities as implanted. Thus,N-type source and drain regions 105 b and 105 c and nitrogen-dopedregions 110 are formed. In the heat treatment, nitrogen implanted intothe polysilicon layer 105 is thermally diffused, while nitrogensegregates in the gate insulating film 111 a. Thus, a gate insulatingfilm 111 having the nitrogen-doped region 110 is formed.

As to an effect of the eleventh embodiment, an interfacial level in theinterface between the polysilicon layer 105 and the gate insulating film111 is reduced since nitrogen segregates in the gate insulating filmllla provided under the channel region 105 a. Thus, it is possible toimprove reliability of the gate insulating film 111. Namely, it ispossible to inhibit hot carriers generated on a drain end from beingtrapped in the gate insulating film 111 by such reduction of theinterfacial level, thereby effectively improving hot carrier resistance.Further, the source/drain regions are also doped with nitrogen, wherebyit is possible to prevent occurrence of a punch-through phenomenoncaused by diffusion of the impurity forming the source/drain regions.

With reference to FIG. 91, a PMOS-TFT according to a twelfth embodimentof the present invention is now described. In the twelfth embodiment,nitrogen-doped regions 110 exist in a gate electrode 120 and a gateinsulating film 111 provided under a channel region 105 a. FIG. 92 showsan impurity profile in a section taken along the line a—a in FIG. 91.Referring to FIG. 92, it is understood that nitrogen is deposited in aportion of the gate insulating film 111 located under the channel region105 a.

With reference to FIGS. 93 to 96, a fabrication process for the PMOS-TFTaccording to the twelfth embodiment shown in FIG. 91 is now described.First, an insulating film 102 is formed on a semiconductor substrate101, and a polysilicon layer 120 a is formed on the insulating film 102in a thickness of about 2000 Å by CDV, as shown in FIG. 93. Arsenic ision-implanted into the polysilicon layer 120 a under conditions of 10keV and 2×10¹⁵/cm². Thereafter boron fluoride is ion-implanted into thepolysilicon layer 120 a, as shown in FIG. 94. The polysilicon layer 120a is patterned in the form of a gate electrode by photolithography andanisotropic etching, thereby forming a gate electrode 120 b shown inFIG. 95.

Then, a gate insulating film 111 a of about 100 Å in thickness is formedby thermal oxidation, and a polysilicon layer (not shown) is formed onthe gate insulating film 111 a in a thickness of about 2000 Å by CDV. Inorder to control a threshold voltage, arsenic is ion-implanted into thepolysilicon layer under conditions of 30 keV and 1×10¹² to 1×10¹³/cm².Then, the polysilicon layer is patterned in a prescribed shape byphotolithography and anisotropic etching, thereby forming a polysiliconlayer 105 shown in FIG. 95.

Then, a resist film 107 is formed on a channel region byphotolithography, as shown in FIG. 96. The resist film 107 is employedas a mask to ion-implant boron fluoride into the polysilicon layer 105under conditions of 30 keV and 4×10¹⁵/cm². Heat treatment is carried outat 850° C. for about 20 minutes, thereby activating the impurities asimplanted. Thus, P-type source and drain regions 105 b and 105 c shownin FIG. 91 are formed. In this heat treatment, nitrogen implanted intothe gate electrode 120 is thermally diffused, while nitrogen segregatesin the gate insulating film 111 a. Thus, the gate insulating film 111having the nitrogen-doped region 110 is formed as shown in FIG. 91.

As to an effect of the twelfth embodiment, the gate electrode 120 isdoped with nitrogen, whereby it is possible to prevent diffusion ofboron in the heat treatment for impurity activation. Thus, it ispossible to prevent boron from entering the channel region 105 a throughthe gate insulating film 111. Further, the gate electrode 120 is dopedwith nitrogen and thereafter heat treated, whereby nitrogen is depositedin the gate insulating film 111. Consequently, it is possible tosuppress occurrence of an interfacial level in the gate insulating film111 caused by hot carrier injection, similarly to the tenth embodiment.Thus, it is possible to improve the gate insulating film 111 inreliability.

An NMOS-TFT according to a thirteenth embodiment of the presentinvention is now described. While the present invention is applied to aPMOS-TFT according to the twelfth embodiment, the same is applied to anNMOS-TFT in the thirteenth embodiment. In order to form such anNMOS-TFT, the conductivity types of the implanted impurities may bereversed to those employed for forming the PMOS-TFT in FIG. 91. Namely,the gate electrode 120 and the source and drain regions 105 b and 105 care doped with an N-type impurity, while the channel region 105 a isdoped with a P-type impurity. FIG. 97 shows an impurity profile of adepth direction in a section taken along the line a—a in FIG. 91 in acase of forming the TFT shown in FIG. 91 as an N-channel type one.Referring to FIG. 97, it is understood that nitrogen is deposited in thegate insulating film 111 located under the channel region 105 a.

A fabrication process for the NMOS-TFT according to the thirteenthembodiment is basically substantially identical to that for the PMOS-TFTaccording to the twelfth embodiment. However, it is necessary to employion species which are of reverse conductivity types to those employedfor forming the PMOS-TFT.

Also in the thirteenth embodiment, the gate electrode 120 is doped withnitrogen similarly to the twelfth embodiment, whereby it is possible toprevent arsenic contained in the gate electrode 120 from being diffusedand injected into the gate insulating film 111 in heat treatment foractivating the impurities. Nitrogen is deposited in the gate insulatingfilm 111 in this heat treatment, whereby it is possible to preventoccurrence of an interfacial level in the gate insulating film 111caused by hot carrier injection. Thus, it is possible to improve thegate insulating film 111 in reliability.

With reference to FIGS. 98 to 100, a dual gate CMOS-TFT according to afourteenth embodiment of the present invention is now described.According to the fourteenth embodiment, an insulating film 102 is formedon a semiconductor substrate 101. In the fourteenth embodiment, a gateelectrode of a PMOS-TFT is formed by a three-layer structure of anon-doped polysilicon layer 125, a tungsten silicide layer 126 and aP-type polysilicon layer 127. Further, a gate electrode of an NMOS-TFTis formed by a three-layer structure of the non-doped polysilicon layer125, the tungsten silicide layer 126 and an N-type polysilicon layer130. A gate insulating film 128 is formed to cover the P-typepolysilicon layer 127, and a polysilicon layer 129 having a channelregion 129 a, a P-type source region 129 b and a P-type drain region 129c is formed on the gate insulating film 128. On the other hand, a gateinsulating film 131 is formed on the N-type polysilicon layer 130, and apolysilicon layer 132 having a channel region 132 a, an N-type sourceregion 132 b and an N-type drain region 132 c is formed on the gateinsulating film 131.

Nitrogen-doped regions 110 exist in the tungsten silicide layer 126, theP-type polysilicon layer 127, the gate insulating film 128, the N-typepolysilicon layer 130 and the gate insulating film 131. FIG. 101 showsan impurity profile in a section taken along the line a—a in FIG. 99,and FIG. 102 shows an impurity profile in a section taken along the lineb—b in FIG. 99. Referring to FIG. 101, peaks of concentrationdistribution of nitrogen exist in the interface between the P-typesilicon layer 127 and the tungsten silicide layer 126, and the gateinsulating film 128 in the gate electrode of the PMOS-TFT. Referring toFIG. 102, peaks of concentration distribution of nitrogen exist in theinterface between the N-type polysilicon layer 130 and the tungstensilicide layer 126, and the gate insulating film 128 in the gateelectrode of the NMOS-TFT.

With reference to FIGS. 103 to 108, a fabrication process for the dualgate CMOS-TFT according to the fourteenth embodiment shown in FIG. 98 isnow described.

First, an insulating film 102 is formed on a semiconductor substrate101, and a polysilicon layer 125 a is formed on the insulating film 102in a thickness of about 500 Å by CDV, as shown in FIG. 103. Then, atungsten silicide layer 126 a is formed on the polysilicon layer 125 ain a thickness of about 500 Å by sputtering. A polysilicon layer 135 isformed on the tungsten silicide layer 126 a in a thickness of about 1000Å.

Then, nitrogen is ion-implanted into a portion close to the interfacebetween the polysilicon layer 135 and the tungsten silicide layer 126 a,as shown in FIG. 104. According to this embodiment, ion implantationconditions for nitrogen may be set at about 40 keV and 2×10¹⁵/cm².

Then, a region for forming a PMOS-TFT is covered with a resist film (notshown), and this resist film is employed as a mask to ion-implantarsenic into a region for forming an NMOS-TFT. Then, the region forforming an NMOS-TFT is covered with a resist film (not shown), and thisresist film is employed as a mask to ion-implant boron fluoride into theregion for forming a PMOS-TFT. FIG. 105 shows a sectional structure ofthe TFT after such ion implantation.

Thereafter the polysilicon layer 135, the tungsten silicide layer 126 aand the non-doped polysilicon layer 125 a are patterned in the form ofgate electrodes. A gate oxide film of about 100 Å in thickness is formedby thermal oxidation, and a polysilicon layer is formed in a thicknessof about 2000 Å by CDV. Thereafter ion implantation is carried out oneach of the regions for forming a PMOS-TFT and an NMOS-TFT forcontrolling a threshold voltage, and thereafter the polysilicon layer ispatterned. Thus, polysilicon layers 129 and 132 are formed as shown inFIG. 106.

Then, a resist film 140 (see FIG. 107) is formed on a region of theNMOS-TFT excluding source and drain regions 132 b and 132 c. The resistfilm 140 is employed as a mask to ion-implant arsenic into thepolysilicon layer 135 under conditions of 30 keV and 4×10¹⁵/cm². Heattreatment is carried out at 850° C. for about 20 minutes, therebyactivating the arsenic ions. Thus, the source and drain regions 132 band 132 c of the NMOS-TFT are formed. Thereafter the resist film 140 isremoved.

Then, a resist film 141 is formed on a region of the PMOS-TFT excludingsource and drain regions 129 b and 129 c. This resist film 141 isemployed as a mask to ion-implant boron fluoride into the source anddrain regions 129 b and 129 c under conditions of 30 keV and 4×10¹⁵/cm².FIG. 108 is a plan view showing the TFT in this stage. Thereafter heattreatment is carried out at 850° C. for about 20 minutes, therebyactivating the boron ions. Thus, the source region 129 b of the PMOS-TFTand the drain region 129 c of the NMOS-TFT are formed.

In the heat treatment step for activating the source/drain regions, theimpurities contained in the gate electrodes are also diffused. However,it is possible to suppress diffusion of boron and arsenic in thetungsten silicide layer 126, since nitrogen is doped in portions closeto the interfaces between the tungsten silicide layer 126 and thepolysilicon layers 127 and 130. Consequently, it is possible to suppressfluctuation of a threshold voltage caused by change in working functionsof the gate electrodes.

With reference to FIG. 109, a stack gate type flash EEPROM according toa fifteenth embodiment of the present invention is now described. In thestack gate type flash EEPROM according to the fifteenth embodiment, adrain region 208 and a source region 209 are formed on a main surface ofa silicon substrate 201 at a prescribed space,to define a channel region215 therebetween. A floating gate electrode 221 is formed on the channelregion 215 through an oxide film 220. A control gate electrode 205 isformed on the floating gate electrode 221 through an interlayerinsulating film 222. Side wall oxide films 206 are formed on sidesurfaces of the floating gate electrode 221 and the control gateelectrode 205. A smooth coating film 212 is formed to cover the sidewall oxide films 206 and the control gate electrode 205. A titaniumalloy film 213 is formed on the smooth coating film 212, and an aluminumalloy wiring layer 214 is formed on the titanium alloy film 213. Acontact hole 212 a is provided in the smooth coating film 212. Thetitanium alloy film 213 is electrically connected to the drain region208 in this contact hole 212 a.

The titanium alloy film 213 is made of titanium nitride, for example.The oxide film 220 has a thickness of about 100 Å. The floating gateelectrode 221 has a thickness of about 1000 Å. The interlayer insulatingfilm 222 is formed by a composite film of a nitride film and an oxidefilm, in a thickness of about 200 Å. Nitrogen-doped regions 219 exist inthe oxide film 220, the polysilicon film 221 and the interlayerinsulating film 222. FIG. 110 shows an impurity profile of a depthdirection in the control electrode 205, the interlayer insulating film222, the floating gate electrode 221 and the oxide film 220 provided inthe flash EEPROM shown in FIG. 109.

With reference to FIGS. 111 to 124, a fabrication process for the stackgate type flash EEPROM according to the fifteenth embodiment shown inFIG. 109 is now described. First, a well region (not shown) and elementisolation oxide film (not shown) are formed on prescribed regions of aP-type silicon substrate 201. Thereafter an oxide film 220 a of about100 Å in thickness is formed on the overall surface. A polysilicon film221 a of about 1000 Å in thickness is formed on the oxide film 220 a.

Then, nitrogen is ion-implanted into the polysilicon film 221 a underconditions of 5 keV and 4×10¹⁵/cm², as shown in FIG. 112. At this time,a projection range R_(P) of nitrogen is set to come to a position of thepolysilicon film 221 a upward beyond a position separated by 5×ΔR_(P)from the interface between the polysilicon film 221 a and the oxide film220 a toward the polysilicon film 221 a, assuming that ΔR_(P) representsits standard deviation. If the projection range R_(P) is set downwardbeyond the said position, the oxide film 220 a may be damaged bynitrogen implantation.

Then, boron is ion-implanted into the polysilicon film 221 a underconditions of 20 keV and 4×10¹⁵/cm², as shown in FIG. 114. Thereafter aninterlayer insulating film 222 a which is formed by a composite film ofan oxide film and a nitride film is formed on the polysilicon film 221 ain a thickness of about 200 Å, as shown in FIG. 115. Thereafter apolysilicon film 205 a is formed on the interlayer insulating film 222 ain a thickness of about 2500 Å.

Then, resist films 225 are formed on prescribed regions of thepolysilicon film 205 a, as shown in FIG. 116. These resist films 225 areemployed as masks to carry out anisotropic etching, thereby patterningthe polysilicon film 205 a, the interlayer insulating film 222 a, thepolysilicon film 221 a and the oxide film 220 a. Thus, a control gateelectrode 205, an interlayer insulating film 222 b, a floating gateelectrode 221 b and an oxide film 220 b are formed as shown in FIG. 117.Thereafter the resist films 225 are removed.

Then, a resist film 226 is formed to cover a portion for serving as asource region of a memory cell, as shown in FIG. 118. The resist film226 and the control gate electrode 205 are employed as masks, toion-implant arsenic into the main surface of the silicon substrate 201under conditions of 35 keV and 5×10¹⁵/cm². Thereafter the resist film226 is removed.

Then, a resist film 227 is formed to cover a portion for serving as adrain region of the memory cell, as shown in FIG. 119. The resist film227 and the control gate electrode 205 are employed as masks toion-implant arsenic into the main surface of the silicon substrate 201under conditions of 35 keV and 5×10¹⁵/cm². Thereafter the resist film227 is removed.

Then, an oxide film 206 a is formed on the overall surface in athickness of about 2000 Å, as shown in FIG. 120. Anisotropic reactiveion etching is carried out on the oxide film 206 a, thereby forming sidewall oxide films 206 as shown in FIG. 121. Each side wall oxide film 206as formed has a width of about 2000 Å in a channel length direction.Namely, the width in the channel length direction is substantiallyidentical to the thickness of the oxide film 206 a shown in FIG. 120.Thus, it is possible to readily control the width of each side walloxide film 206 in the channel length direction by adjusting thethickness of the oxide film 206 a. After formation of the side walloxide films 206, heat treatment is carried out at 850° C. for about 60seconds, thereby activating the impurities as implanted. Thus, a sourceregion 209 and a drain region 208 are formed. Due to this heattreatment, boron and nitrogen implanted into the floating gate electrode221 b are diffused. At this time, nitrogen is diffused in advance ofboron, whereby only nitrogen is deposited in the oxide film 220 b andthe interlayer insulating film 222 b. Thus, nitrogen-doped regions 219are formed in the oxide film 220, the floating gate electrode 221 andthe interlayer insulating film 222.

Then, a smooth coating film 212 is formed in a thickness of 5000 to15000 Å by CDV, as shown in FIG. 122. Thereafter heat treatment iscarried out under a temperature condition of 800 to 1000° C. by a reflowmethod, thereby flattening the surface of the smooth coating film 212.The smooth coating film 212 is formed by a PSG film, a BPSG film, anitride film, a non-doped oxide film or a layered film of these films,for example.

Then, a contact hole 212 a of about 0.6 to 1.5 μm in diameter is formedin a portion of the smooth coating film 212 located on the drain region208, as shown in FIG. 123. Thereafter a titanium alloy film 213consisting of titanium nitride is formed on a side surface of thecontact hole 212 a and the smooth coating film 212 for electricalconnection with the drain region 208. Finally, an aluminum alloy wiringlayer 214 of about 1000 Å in thickness is formed on the titanium alloyfilm 213 by sputtering. The titanium alloy film 213 and the aluminumalloy wiring layer 214 are patterned by photolithography and dryetching. Thus formed is a bit line consisting of the titanium alloy film213 and the aluminum alloy wiring layer 214, which is electricallyconnected with the drain region 208. Thus, the flash EEPROM according tothe fifteenth embodiment shown in FIG. 109 is completed. Implantationinto the source/drain regions may be simultaneously carried out in thestep shown in FIG. 117 through the mask of the resist film 226.

In the flash EEPROM according to the fifteenth embodiment, nitrogen ision-implanted into the floating gate electrode 221 so that nitrogen isdeposited in the oxide film 220 and the interlayer insulating film 222by subsequent thermal diffusion, whereby no doping of hydrogen is causeddissimilarly to RTN treatment. Due to the effect of nitrogen depositedin the oxide film 220, further, it is possible to suppress trapping andoccurrence of an interfacial level caused by hot carrier injection,trapping and occurrence of an interfacial level caused by holesresulting from interband tunneling in a write or erase operation throughF-N tunneling. Further, no deterioration of the oxide film 220 is causedby doping of hydrogen, whereby the oxide film 220 is improved inreliability and it is possible to reduce a probability of occurrence ofan initial failure in the flash EEPROM. In addition, the interlayerinsulating film 222 is also improved in reliability by nitrogencontained therein. Upon such improvement in reliability of theinterlayer insulating film 222, it is possible to reduce the film 222 inthickness, whereby a capacitance C_(FC) between the control gateelectrode 205 and the floating gate electrode 221 can be increased.Namely, a higher electric field is applied to the channel to improvecurrent drivability when the coupling ratio is large, regardless of thepotential which is applied to the control gate electrode 205. Thus, itis possible to reduce the potential which is applied to the control gateelectrode 205, thereby reducing the voltage of a power source.

Further, the floating gate electrode 221 is doped with nitrogen, wherebydiffusion of boron is suppressed. Thus, it is possible to inhibit boronfrom punching through the channel region 215 and being injected into theoxide film 220, thereby effectively suppressing fluctuation of thethreshold voltage. In addition, nitrogen is doped by ion implantation inthe fabrication process for the flash EEPROM according to the fifteenthembodiment, whereby the silicon substrate 201 is not exposed to abrupttemperature change, dissimilarly to RTN treatment. Thus, it is alsopossible to suppress occurrence of stripy defects.

In RTN treatment, nitrogen may be diffused over a wide range of thesilicon substrate 201 since it is necessary to apply heat in nitrogendoping. In the fabrication process according to this embodiment,however, it is not necessary to carry out a heat treatment step inimplantation of nitrogen since nitrogen is doped by ion implantation.Thus, the heat treatment can be effectively carried out after patterningof the gate electrode 221, not to diffuse nitrogen in the source region209 and the drain region 208.

While boron is ion-implanted into the polysilicon film 221 a for formingthe floating gate electrode 221 which is doped in a P type in thefabrication process for the flash EEPROM according to the fifteenthembodiment, arsenic ions may alternatively be implanted into an N-typedoped polysilicon film which is doped with phosphorus by about5×10²⁰/cm³ or a polysilicon film under conditions of about 50 keV and4×10¹⁵/cm², thereby forming the floating gate electrode 221.

With reference to FIG. 125, a stack gate type flash EEPROM according toa sixteenth embodiment of the present invention is now described. In theflash EEPROM according to the sixth embodiment, a source region 209 anda drain region 208 are formed on a main surface of a P-type siliconsubstrate 201 at a prescribed space, to hold a channel region 215therebetween. A floating gate electrode 203 is formed on the channelregion 215 through an oxide film 202. A control gate electrode 223 isformed on the floating gate electrode 203 through an interlayerinsulating film 222 which is formed by a composite film of a nitridefilm and an oxide film. The interlayer insulating film 222 has athickness of about 200 Å. The control gate electrode 223 is formed by apolysilicon film, and has a thickness of about 2500 Å. Nitrogen-dopedregions 219 exist in the interlayer insulating film 222 and the controlgate electrode 223.

With reference to FIGS. 126 to 128, a fabrication process for the flashEEPROM according to the sixteenth embodiment shown in FIG. 125 is nowdescribed. First, a well region and element isolation oxide film (notshown) are formed on prescribed regions of a P-type silicon substrate201, and thereafter an oxide film 202 a of about 100 Å in thickness, apolysilicon film 203 a of about 1000 Å in thickness, an interlayerinsulating film 222 a of about 200 Å in thickness which is formed by acomposite film of an oxide film and a nitride film, and a polysiliconfilm 223 a of about 2500 Å in thickness are successively formed on theoverall surface, as shown in FIG. 126.

Then, nitrogen is ion-implanted into the polysilicon film 223 a underconditions of about 10 keV and 4×10¹⁵/cm², as shown in FIG. 127. At thistime, a projection range R_(P) of nitrogen is set to come to a positionof the polysilicon film 223 a upward beyond a position separated by5×ΔR_(P) from the interface between the polysilicon film 223 a and theoxide film 222 a toward the polysilicon film 223 a, assuming that ΔR_(P)represents its standard deviation, similarly to the fabrication processfor the fifteenth embodiment (see FIG. 113).

Then, boron is ion-implanted into the polysilicon film 223 a underconditions of 20 keV and 4×10¹⁵/cm², as shown in FIG. 128. Thereafterthe flash EEPROM as shown in FIG. 125 is completed, through afabrication process which is similar to that for the fifteenthembodiment described above with reference to FIGS. 116 to 124. In a heattreatment step for activating the impurities in the sixteenthembodiment, however, nitrogen which is doped in the control gateelectrode 223 is deposited in the interlayer insulating film 222. Whileboron is ion-implanted into the polysilicon film 223 a for forming thecontrol gate electrode 223 which is doped in a P type in the sixteenthembodiment, a doped polysilicon film which is doped with phosphorus byabout 5×10²⁰/cm³ or an N-type polysilicon film which is formed byimplanting arsenic ions into a polysilicon film under conditions ofabout 50 keV and 4×10¹⁵/cm² may alternatively be employed to form thecontrol gate electrode 223.

Also in the sixteenth embodiment, it is possible to improve theinterlayer insulating film 222 in reliability while reducing the voltageof a power source for the element, similarly to the fifteenthembodiment. Further, it is possible to prevent diffusion of boron whichis doped in the control gate electrode 223 in heat treatment byimplanting nitrogen into the control gate electrode 223, therebypreventing boron from being injected into the interlayer insulating film222.

With reference to FIG. 129, a stack gate type flash EEPROM according toa seventeenth embodiment of the present invention is now described. Inthe seventeenth embodiment, the fifteenth and sixteenth embodiments arecombined with each other.

With reference to FIGS. 130 and 131, a fabrication process for the flashEEPROM according to the seventeenth embodiment is now described.Elements up to a polysilicon film 223 a shown in FIG. 113 are formedthrough a process which is similar to that of the fifteenth embodimentshown in FIGS. 111 to 115. Nitrogen is ion-implanted into thepolysilicon film 223 a under conditions of about 10 keV and 4×10¹⁵/cm².Then, boron is ion-implanted into the polysilicon film 223 a underconditions of about 20 keV and 4×10¹⁵/cm², as shown in FIG. 131.Thereafter the flash EEPROM shown in FIG. 129 is completed through stepswhich are similar to those of the fifteenth embodiment shown in FIGS.116 to 124. In a heat treatment step for activating impurities in theseventeenth embodiment, however, nitrogen which is doped in the controlelectrode 223 b is also deposited in the interlayer insulating film 222b simultaneously with nitrogen which is doped in the floating gateelectrode 221 b and deposited in the oxide film 220 b and the interlayerinsulating film 222 b. While the floating gate electrode 221 b and thecontrol electrode 223 b are of P types in the above description, thesame may alternatively be of N types. In this case, a doped polysiliconfilm which is doped with phosphorus by about 5×10²⁰/cm³¹ , or an N-typepolysilicon film obtained by ion-implanting arsenic into a polysiliconfilm under conditions of about 50 keV and 4×10¹⁵/cm² is employed.

Effects of the stack gate type flash EEPROM according to the seventeenthembodiment are similar to those of the fifteenth and sixteenthembodiments as described above.

With reference to FIG. 132, a buried channel type flash EEPROM accordingto an eighteenth embodiment of the present invention is now described.In the buried channel type flash EEPROM according to the eighteenthembodiment, a source region 209 and a drain region 208 are formed on amain surface of a P-type silicon substrate 201 at a prescribed space, tohold a channel region 215 therebetween. A floating gate electrode 203 isformed on the channel region 215 through an oxide film 202 having asmall thickness of about 100 Å. A control gate electrode 205 is formedon the floating gate electrode 203 through an interlayer insulating film204. Side wall oxide films 206 are formed on side surfaces of thefloating gate electrode 203 and the control gate electrode 205. AnN-type impurity layer 217 is formed on the channel region 215, and aP-type impurity layer 218 is formed under the N-type impurity layer 217.A nitrogen-doped region 219 is formed on a main surface of the N-typeimpurity layer 217.

With reference to FIGS. 133 to 136, a fabrication process for the buriedchannel type flash EEPROM according to the eighteenth embodiment is nowdescribed. First, a well region (not shown) and element isolation oxidefilm (not shown) are formed on prescribed regions of a P-type siliconsubstrate 201. As shown in FIG. 133, nitrogen is ion-implanted into thesilicon substrate 201 in such a range that a depth from the main surfaceof the silicon substrate 201 is smaller than about 500 Å. Then, anN-type impurity such as arsenic or phosphorus is ion-implanted into thesilicon substrate 201 in such a range that a depth from the main surfaceof the silicon substrate 201 is not more than about 500 Å, as shown inFIG. 134. Further, a P-type impurity such as boron is implanted in sucha range that a depth from the main surface of the silicon substrate 201is at least about 500 Å, as shown in FIG. 135. Namely, nitrogen isimplanted at such energy that the range of nitrogen is smaller than thatof arsenic.

Then, an oxide film 202 a of about 100 Å in thickness, a polysiliconfilm 203 a of about 1000 Å in thickness, an interlayer insulating film204 a of about 200 Å in thickness, which is formed by a composite filmof an oxide film and a nitride film, and a polysilicon film 205 a ofabout 2500 Å in thickness are successively formed on the overallsurface, as shown in FIG. 136. Thereafter the flash EEPROM according tothe eighteenth embodiment is completed through a process which issimilar to that for the fifteenth embodiment described above withreference to FIGS. 116 to 124. In the eighteenth embodiment, however,the impurities implanted into the N-type impurity layer 217 and theP-type impurity layer 218 are activated and the nitrogen-doped region219 is simultaneously formed in a heat treatment step which is similarto that of the fifteenth embodiment. The N-type impurity layer 217 isformed to cover the nitrogen-doped region 219 by the aforementionedimpurity ion implantation conditions, whereby no defects are caused byion implantation of nitrogen in junction planes of the N-type impuritylayer 217 and the P-type impurity layer 218. Thus, no junction leakagecurrent is increased, whereby no damage is caused by implantation ofnitrogen.

According to the eighteenth embodiment, a region shallower than theN-type impurity layer 217 is doped with nitrogen, whereby diffusion ofarsenic is suppressed. Further, it is also possible to prevent diffusionof boron in the P-type impurity layer 218. Thus, the N-type impuritylayer 217 can be formed in a small thickness, whereby it is possible tosuppress a punch-through phenomenon in the buried channel type flashEEPROM. Further, it is possible to control the thickness of the N-typeimpurity layer 217 at a desired value by controlling nitrogenimplantation conditions.

With reference to FIG. 137, a stack gate type flash EEPROM according toa nineteenth embodiment of the present invention is now described. Inthe nineteenth embodiment, a nitrogen-doped region 230 is formed in adrain region 208. FIG. 138 shows an impurity profile of a depthdirection in the drain region 208 of the flash EEPROM shown in FIG. 137.Referring to FIG. 138, it is understood that a junction plane of thedrain region 208 is not doped with nitrogen, but the nitrogen-dopedregion 230 exists in the drain region 208 which is formed by doping witharsenic.

With reference to FIGS. 139 to 141, a fabrication process for the flashEEPROM according to the nineteenth embodiment shown in FIG. 137 is nowdescribed. First, a well region (not shown) and element isolation oxidefilm (not shown) are formed on prescribed regions of a P-type siliconsubstrate 201. As shown in FIG. 139, an oxide film 202 a of about 100 Åin thickness, a polysilicon film 203 a of about 1000 Å in thickness, aninterlayer insulating film 204 a of about 200 Å in thickness, which isformed by a composite film of an oxide film and a nitride film, apolysilicon film 205 a of about 2500 Å in thickness, and an oxide film207 a of about 1000 Å in thickness are successively formed on theoverall surface, as shown in FIG. 139.

Then, the oxide film 202 a, the polysilicon film 203 a, the interlayerinsulating film 204 a, the polysilicon film 205 a, and the oxide film207 a are patterned in the form of a gate electrode, thereby forming anoxide film 202, a floating gate electrode 203, an interlayer insulatingfilm 204, a control gate electrode 205 and an oxide film 207, as shownin FIG. 140. Thereafter a source forming region is covered with a resistfilm 225. The resist film 225 and the oxide film 207 are employed asmasks to ion-implant nitrogen into a drain forming region underconditions of about 10 keV and 8×10¹⁵/cm².

Then, the resist film 225 and the oxide film 207 are again employed asmasks to ion-implant arsenic under conditions of 35 keV and 5×10¹⁵/cm²,as shown in FIG. 141. Namely, nitrogen is implanted at such energy thatits range is smaller than that of arsenic. Thereafter the resist film225 is removed. Subsequent fabrication steps are identical to those ofthe fifteenth embodiment shown in FIGS. 119 to 124. In the fabricationprocess for the nineteenth embodiment, however, the impurities implantedinto the source region 209 and the drain region 208 are activated andthe nitrogen-doped region 230 is simultaneously formed in the heattreatment step in the fifteenth embodiment. Due to the aforementionedimpurity ion implantation conditions, the drain region 208 is formed tocover the nitrogen-doped region 230. Thus, no defects are caused by ionimplantation of nitrogen in a junction plane between the drain region208 and the silicon substrate 201, and hence no junction leakage currentis increased. Thus, no damage is caused by implantation of nitrogen.

According to the nineteenth embodiment, the drain region 208 is dopedwith nitrogen, whereby it is possible to prevent diffusion of arsenicwhich is implanted into the drain region 208 in the heat treatment step.Therefore, it is possible to shallowly form the P-N junction planebetween the drain region 208 and the silicon substrate 201 Thus, it ispossible to suppress a short channel effect such as a punch-throughphenomenon. Due to such suppression of a short channel effect, further,it is possible to refine the element.

Diffusion of arsenic which is implanted into the drain region 208 isthus suppressed by nitrogen which is doped in the drain region 208,whereby an overlap region between the oxide film 202 and the drainregion 208 caused by transverse diffusion of arsenic is reduced. Thus, acapacitance C_(FS) between the control gate electrode 205 and the drainregion 208 is reduced. Therefore, it is possible to increase thecoupling ratio C_(FC)/C_(TOTAL), whereby potential difference betweenpotentials V_(CG) and V_(FG) of the control gate electrode 205 and thefloating gate electrode 203 is reduced. Namely, a higher electric fieldis applied to the channel region 215 to improve current drivability whenthe coupling ratio is large, regardless of the potential which isapplied to the control gate electrode 205. Thus, it is possible toreduce the voltage V_(CG) which is applied to the control gate electrode205 as the coupling ratio is increased, thereby reducing the voltage ofa power source.

With reference to FIG. 142, a stack gate type flash EEPROM according toa twentieth embodiment of the present invention is now described.According to the twentieth embodiment, a nitrogen-doped region 231 isformed in a source region 209. As to a fabrication process for the flashEEPROM according to the twentieth embodiment, the nitrogen doping stepof the nineteenth embodiment may be carried out in advance of a sourceimplantation step. Also in the twentieth embodiment, nitrogen isimplanted at such energy that its range is smaller than that of arsenic,similarly to the aforementioned fabrication process for the nineteenthembodiment. An effect similar to that of the nineteenth embodiment canbe attained in the twentieth embodiment.

With reference to FIG. 143, a stack gate type flash EEPROM according toa twenty-first embodiment of the present invention is now described. Inthe twenty-first embodiment, the nineteenth and twentieth embodimentsare combined with each other. Namely, a nitrogen-doped region 230 isformed in a drain region 208, and a nitrogen-doped region 231 is formedin a source region 209.

As to a fabrication process for the flash EEPROM according to thetwenty-first embodiment, the nitrogen doping step in the nineteenthembodiment may be carried out after a step of patterning a gateelectrode (see FIG. 144).

According to the twenty-first embodiment, the drain region 208 and thesource region 209 are provided with the nitrogen-doped regions 230 and231 respectively, whereby the effect of the nineteenth or twentiethembodiment further remarkably appears. In the twenty-first embodiment,nitrogen may be ion-implanted without providing an oxide film 207,thereby doping a control gate electrode 205 also with nitrogen.

In the semiconductor device according to the first aspect of the presentinvention, as hereinabove described, it is possible to suppressdiffusion of the impurity which is introduced into the gate electrode bydoping the gate electrode with nitrogen. Thus, it is possible to inhibitthe impurity from being mixed into the gate insulating film and punchingthrough the gate insulating film. Further, it is possible to improve thegate insulating film in reliability and hot carrier resistance by dopingthe gate insulating film with nitrogen.

In the method of fabricating a semiconductor device according to thesecond aspect of the present invention, on the other hand, heattreatment is carried out after nitrogen is ion-implanted into the gateinsulating film to deposit nitrogen in the gate insulating film, wherebyit is possible to form the gate insulating film doped with no hydrogenwithout damaging the same.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A method of fabricating a semiconductor device,comprising the steps of: forming an insulating layer on a semiconductorsubstrate; forming an electrode layer on said insulating film;ion-implanting nitrogen into said electrode layer; implanting animpurity into said electrode layer after the step of ion-implantingnitrogen, whereby no heat treating occurs between the steps ofion-implanting nitrogen and implanting said impurity; and carrying outheat treatment after said steps of implanting nitrogen and said impurityinto said electrode layer, thereby depositing nitrogen in saidinsulating film.
 2. A method of fabricating a semiconductor device inaccordance with claim 1, wherein a projection range R_(P) of saidnitrogen in said step of ion-implanting said nitrogen is set to at sucha position that a distance from an interface between said insulatingfilm and said electrode layer toward said electrode layer is at least5×ΔR_(P), assuming that ΔR_(P) represents its standard deviation.
 3. Amethod of fabricating a semiconductor device, comprising the steps of:forming a first insulating layer on a semiconductor substrate; formingan electrode layer on said first insulating film; ion-implantingnitrogen into said electrode layer; implanting an impurity into saidelectrode layer after the step of ion-implanting nitrogen, whereby noheat treating occurs between the steps of ion-implanting nitrogen andimplanting said impurity; forming a second insulating layer on saidelectrode layer after said steps of implanting said nitrogen and saidimpurity; and carrying out heat treatment after said step of formingsaid second insulating layer, thereby depositing nitrogen in saidinsulating film.
 4. A method of fabricating a semiconductor device,comprising the steps of: forming a gate electrode on a semiconductorsubstrate; implanting nitrogen ions into said semiconductor substratefor forming nitrogen-doped regions in said semiconductor substrate tohold said gate electrode therebetween; and implanting impurity ionshaving a range being larger than that of said nitrogen ions into saidsemiconductor substrate after the step of implanting nitrogen ions,whereby no heat treating occurs between the steps of implanting nitrogenions and implanting impurity ions, thereby forming source/drain regionsincluding said nitrogen-doped regions.
 5. A method of fabricating asemiconductor device in accordance with claim 4, wherein said nitrogenions are implanted into said semiconductor substrate at an angle beingsmaller than 90° with respect to a main surface of said semiconductorsubstrate.
 6. A method of fabricating a semiconductor device inaccordance with claim 4, wherein said step of implanting said nitrogenions includes a step of doping said gate electrode with nitrogen, andsaid step of implanting said impurity ions includes a step of dopingsaid gate electrode with an impurity, said method further comprising astep of carrying out heat treatment after said step of implanting saidimpurity into said gate electrode and said semiconductor substrate,thereby depositing nitrogen in said gate insulating film.
 7. A method offabricating a semiconductor device comprising a semiconductor substratehaving a first conductivity type first semiconductor region and a secondconductivity type second semiconductor region, a gate insulating filmbeing formed on said semiconductor substrate, and a gate electrode layerbeing formed on said gate insulating film, said method comprising thesteps of: ion-implanting nitrogen into the overall surface of said gateelectrode layer; introducing a second conductivity type impurity into aportion of said gate electrode layer being formed on said firstsemiconductor region after the step of ion-implanting nitrogen, wherebyno heat treating occurs between the steps of ion-implanting nitrogen andintroducing said second conductivity type impurity; introducing a firstconductivity type impurity into a portion of said gate electrode layerbeing formed on said second semiconductor region; and carrying out heattreatment after said introduction of said nitrogen and said impuritiesinto said gate electrode layer, thereby depositing nitrogen into saidgate insulating film.
 8. A semiconductor device in accordance with claim7, wherein said gate electrode layer consists of a polysilicon layer anda metal silicide film being formed on said polysilicon layer, saidnitrogen is ion-implanted into an interface between said polysiliconfilm and said metal suicide film, said second conductivity type impurityis introduced into a portion of said polysilicon film being formed onsaid first semiconductor region, and said first conductivity typeimpurity is introduced into a portion of said polysilicon film beingformed on said second semiconductor region.
 9. A method of fabricating asemiconductor device having a polysilicon film serving as an activesemiconductor layer, a first insulating film being formed on a firstsurface of said active semiconductor layer, and a gate electrode beingformed through said first insulating film, said method comprising thesteps of: implanting nitrogen ions into said polysilicon film by obliquerotation implantation; and implanting an impurity into source/drainforming regions of said polysilicon film after said step of implantingsaid nitrogen ions, whereby no heat treating occurs between the steps ofion-implanting nitrogen and introducing said second conductivity typeimpurity.
 10. A method of fabricating a semiconductor device, comprisingthe steps of: ion-implanting nitrogen into a gate electrode; forming agate insulating film on said gate electrode after said step ofimplanting said nitrogen ions; forming a polysilicon film serving as anactive semiconductor layer on said gate insulating film, wherein saidactive semiconductor layer is formed by implanting an impurity into saidpolysilicon film after said step of implanting nitrogen ions, whereby noheat treating occurs between the step of implanting nitrogen ions andimplanting said impurity; and carrying out heat treatment after saidsteps of implanting nitrogen and said impurity into said electrodelayer, thereby depositing nitrogen in said insulating film.